I believe that everything have reason to exist . I want to learn why the direct sum exist.
Is there any motivation for the direct sum to exist ?
From a bottom-up perspective, one thing we do with mathematical structures is combine them to make new ones. The most basic way of doing this with e.g. groups and rings is the direct sum. This is how get get vector spaces from scalar fields, after all (up to isomorphism), using coordinates.
In the top-down perspective, one thing we do with mathematical objects is describe their internal structure, which means decomposing them into simpler things (like naturals factoring into primes or representations into irreducibles), and with groups and rings this means decomposing into internal direct sums (among various more general types of reductions, like semidirect products or not-necessarily-split extensions). For instance the fundamental theorem on finitely-generated modules over a PID generalizes the classification of all finite abelian groups which is itself a generalization of the fundamental theorem of arithmetic.
From a categorical point of view, one notices a number of independently motivated operations on mathematical structures in different areas (direct sums of abelian groups and rings, disjoint unions of sets and topological spaces) have the same arrow-theoretic abstraction in terms of universal properties. This leads to the categorical operation of coproduct, a generalization of direct sum.