In what situation do we need to use Substitution Rule? What is the purpose for using Substitution Rule?

Substitution rule is one of the most important rule to solve the integrals problem.

In substitution rule we substitute different variable in such a way that it can simplify the question.

example: when we have to solve "Sin x²" we cannot solve it but

if question is "x Sin x²" we can substitute x² with t and can solve it.

The purpose of substitution rule is to simplify the problem of integrals.