Question

why we need to balance chemical equation\

Answer 1

we need to balance the chemical equation to satisfy the law of conservation of mass which states that mass can neither be created nor destroyed in a chemical reaction

Answer 2

we need to balance chemical equation because to satisfy the law of mass conservation.
law of mass conservation says that the number of electrons in reactant and product side will be same.

it states that mass neither can be created nor be destroyed, only it can transform from one form to another form.