What are some applications of subdirect product? I have studied direct products. I know a few applications of direct products, like group isomorphism, etc. What are some applications of sub-direct product of groups?
 A: Subdirect products arise naturally as intransitive subgroups of $S_n$, where they are subdirect products of the induced actions of the group on its orbits. Similarly for completely reducible linear groups.
A: A powerful tool in group theory is something called the fibre product. This is a special type of subdirect product of the group with itself.

Associated to a short exact sequence $1\rightarrow N\rightarrow H\rightarrow Q\rightarrow 1$, where $\pi(H)=Q$ is the natural map, is the fibre product $P\subset H\times H$,
  $$
P:=\{(h_1, h_2)\mid \pi(h_1)=\pi(h_2)\}.
$$

Clearly the diagonal component $\Delta:=\{(h, h)\mid h\in H\}$ is contained in $P$, so $P$ is a subdirect product.
The first application of fibre products which is know of is that there exist finitely generated subgroups of $F_2\times F_2$ which have insoluble membership problem: Firstly, let $Q$ be finitely presented and have insoluble word problem, then $P$ has insoluble membership problem. To see that $P$ is finitely generated, since $Q$ is ﬁnitely presented we have that $N \subset H$ is ﬁnitely generated as a normal subgroup, and then to obtain a ﬁnite generating set for $P$ one chooses a ﬁnite normal generating set for $N\times\{1\}$ and then appends a generating set for the diagonal $\Delta\cong H=F_2$.
A surprisingly powerful result, called the $1$-$2$-$3$ theorem, gives conditions on the fibre products to be finitely presentable, see:  G. Baumslag, M.R. Bridson, C.F. Miller III, H. Short, Fibre products, non-positive curvature, and decision problems, Comm. Math. Helv. 75 (2000), 457–477.
A super-powerful application of fibre products is the following (the application is the main result of a paper in the Annals of Mathematics, one of the top journals - undisputed top 4 journal, disputed no. 1). If $\Gamma$ is a group then $\widehat{\Gamma}$ denotes its profinite completion.
Question (Grothendieck, 1970). Let $\Gamma_1$ and $\Gamma_2$ be ﬁnitely presented, residually ﬁnite groups and let $u :\Gamma_1\rightarrow \Gamma_2$ be a homomorphism such that $\widehat{u} :\widehat{\Gamma}_1\rightarrow \widehat{\Gamma}_2$ is an isomorphism of proﬁnite groups. Does it follow that $u$ is an isomorphism from $\Gamma_1$ onto $\Gamma_2$?
Theorem (Bridson, Grunewald, 2004, Ann. Math., link). There exists a short exact sequence $1\rightarrow N\rightarrow\Gamma\rightarrow Q\rightarrow 1$ where $\Gamma$ has a whole host of nice properties, and where $P$ and $\Gamma$ are a counter-example to Grothendieck's question: $P$ and $\Gamma$ are finitely presentable, with $\widehat{P}\cong \widehat{\Gamma}$ but $P\not\cong \Gamma$.
A: Typically, when you know that an object is a subdirect product of other objects, you know that it inherits all properties that are passed from products of those objects to their subsets. For example, a subdirect product of abelian groups must be abelian (and the same for any other group identity).
