What is so special about $a*b^{ -1}$ equivalence? This equivalent is used often in group theory. For example, using this equivalnce you prove Lagranges theroem and also this equivalence gives you the cosets and other things. This equivalence also gives you the modulo equivalnce in $\langle \Bbb{Z}, + \rangle$. But most equivalences disjoin the set accordingnly and you can do probably the same things. 
What I really mean is is there any reason for it that is being used so much especially in group theory does it have something or it is just what they came up that fits to group theory? Because most constructions in group theory are made up from this equivalence? Is it the only one it does the job done? The equiv is $a$ is related to $b$ if  $a*b^{-1}$ belongs to $H$ subgrougroup of a given $\langle G,*\rangle$
 A: I presume you mean the equivalence relation induced by a subgroup $H\le G$ defined by $a\sim b$ if and only if $ab^{-1}\in H$. I have two answers for this.
(1) Groups are meant to study symmetry. I think groups are to group actions as potential energy is to kinetic energy: groups act on things. So group actions are of fundamental importance. Often we examine certain types of actions on certain types of objects: linear transformations on a vector space, or automorphisms of a graph, or rotations preserving a polyhedron, or diffeomorphisms of a manifold, and so on. But the most basic is just a group action on a set, with no additional structure involved. This type of thing is used in leveraging symmetry to explore combinatorial counting problems, for example necklace counting. This is  relevant to Polya enumeration, cycle indices, generating functions, and most generally combinatorial species.
Anyway, the most basic building block in the theory of group actions is that of orbits. We can say that every $G$-set is a disjoint union of orbits. The categorical version of the orbit-stabilizer theorem states that every orbit is isomorphic (as a $G$-set, via a $G$-equivariant bijection) to the coset space $G/H$, where $H$ is the stabilizer of any element in the orbit. So coset spaces are natural, and they form a partition of $G$. Partitions of sets correspond to equivalence relations, and the equivalence relation in this case is $aH=bH$, or equivalently $b^{-1}a\in H$. With right actions it's $ab^{-1}\in H$.
(2) Congruence relations are fundamental in abstract algebra for collapsing algebraic structures into quotients (quotient groups, quotient rings, quotient modules, etc.), and by the first isomorphism theorem in the appropriate category this is equivalent to characterizing all of the homomorphic images of an algebraic structure. A congruence relation $\sim$ on a group $G$ is an equivalence relation such that $a\sim b$ and $c\sim d$ imply $ac\sim bd$ for all $a,b,c,d\in G$. One may prove that such a relation is precisely that of $a\sim b$ iff $aN=bN$ for some normal subgroup $N\triangleleft G$.
A: I presume when you say "equivalence $a \cdot b^{-1}$" you mean a relation $a \cdot b^{-1} = 1$ in a group presentation. (We may also consider a more general situation in which we impose the relation $a \cdot b^{-1}$.) It is not clear from your question wheter $a$ and $b$ should be generators or arbitrary expressions, but that doesn't matter too much.
In group theory can we present finitely generated congruences by elements because we can always transform an equation $L = R$ to one of the form $L \cdot R^{-1} = 1$. If you meant for $a$ and $b$ to be arbitrary expressions, then the answer is: because group theorists "optimize" group presentations so that they are of the form $E = 1$ where $E$ is an expression. They could not afford to do it if they worked with semigroups, for instance. This kind of "optimization" can be harmful to students if they are not told how things fit into a more general picture: a group presentation is just a quotient of a free group by a congruence relation. There are billions of ways of defining a congruence relation, of which group theorists particularly like finite presentations, and they use the group structure to always present the relations in the form $E_1 = 1, \ldots, E_n = 1$ – and they just list $E_1, \ldots, E_n$.
If $a$ and $b$ are supposed to be generators, or particular elements of a group (such as in the case of getting $\mathbb{Z}/n$ from $\mathbb{Z}$), then the answer is again similar: the congruence relation generated by $a \cdot b^{-1}$ is just imposing the equation $a = b$, which just means "let us make $a$ and $b$ equal". Obviously, there will be many applications in which we would want to equate two elements. Again, it is an accident of group theory that equating $a$ and $b$ is the same as equating $a \cdot b^{-1}$ and $1$.
The whole thing reminds me a bit of how students of homological algebra are not aware of the connection between exact sequences and coequalizers (because nobody told them): in an abelian category the coequalizer $q : B \to Q$ of $f : A \to B$ and $g : A \to B$ can be "optimized" to an exact sequence
$$A \to B \to Q \to 0 $$
where the arrow $A \to B$ is $f - g$ and the arrow $B \to Q$ is $q$. Again, we have the same idea: rather than equating $f(x) = g(x)$ we equate $f(x) - g(x) = 0$. And the students are only ever shown the exact sequence, which is a shame.
A: For a general algebraic structure ("universal algebra"), a congruence is defined as an equivalence relation compatible with all  operations of the algebra, i.e. $\,a'\equiv a,\ b'\equiv b\,\Rightarrow\, a'\circ b'\equiv a\circ b,\, $ for all operations $\,\circ\,$ (like ring congruences on integers). Such compatibility implies the algebraic operations persist as well-defined operations on the equivalence classes, so the algebraic structure persists on the congruence classes, yielding a quotient algebra of the same type. Equivalently, congruences can be viewed as certain subalgebras of the squared algebra $\rm\:A^2\:,\:$ e.g. see here.
Algebras like groups and rings, where we can normalize $\,a = b\,$ to $\,a\!-\!b = 0\,$ have congruences that are determined by a single congruence class (e.g. an ideal in a ring). This has the effect of collapsing the relationship of congruences with subalgebras from  $\rm\,A^2\,$ down to  $\rm\,A.\,$  Such algebras are called ideal determined varieties and they have been much studied. One basic result is that they are characterized by two properties of their congruences, being $0$-regular and permutable. Below is an excerpt of one paper on related topics that yields an entry point into such literature.

On subtractive varieties iv: Definability of principal ideals.


Paolo Agliano and Aldo Ursini




*Foreword



We have been asked the following questions:



*

*(a) What are ideals in universal algebra good for?



*

*(b) What are subtractive varieties good for?

*(c) Is there a reason to study definability of principal ideals?


Being in the middle of a project in subtractive varieties,
this seems the right place to address them.


To (a). The notion of ideal in general algebra [13], [17], [22] aims
at recapturing some essential properties of the congruence classes of $0$,
for some given constant $0$. It encompasses: normal subgroups, ideals
in rings or operator groups, filters in Boolean or Heyting algebras,
ideals in Banach algebra, in l-groups and in many more classical
settings. In a sense it is a luxury, if one is satisfied with the
notion of "congruence class of $0$". Thus in part this question might
become: Why ideals in rings? Why normal subgroups in groups? Why filters
in Boolean algebras?, and many more. We do not feel like attempting any
answer to those questions. In another sense, question (a) suggests similar
questions: What are subalgebras in universal algebra good for? and many
more. Possibly, the whole enterprise called "universal algebra" is
there to answer such questions?


Having said that, it is clear that the most proper setting for a theory
of ideals is that of ideal determined classes (namely, when mapping a
congruence E to its $0$-class $\,0/E$ establishes a lattice isomorphism between
the congruence lattice and the ideal lattice). The first paper in this
direction [22] bore that in its title.


It comes out that -- for a variety V -- being ideal determined is the
conjunction of two independent features:



*

*V has $\,0$-regular congruences, namely for any congruences $\rm\,E,E'$
of any member of $V,$ from  $\,\rm 0/E = 0/E'$ it follows  $\rm\,E = E'$.





*V has $0$-permutable congruences, namely for any congruences $\,\rm E,E'$
of any member of $V,$ if  $\,\rm 0 \ E\ y \ E'\, x,\,$ then for some $\rm z,\  0\ E'\, z\ E\ x.$

