Question

Why is it important to balanve a chemical reaction

Answer 1

Because if you do you can know how many atoms react and are produced. More importantly, this means that you can figure out how many moles of each element are used and come out.

Answer 2

It is important because you are usually interested in finding of how efficient your processes are, and only by knowing the stoichiometry will you know what the expected yields of products and the correct ratio of reactants will be.

As a chemist, you do not want to waste your chemicals, you strive to improve your processes to increase the yields so that you minimize the wastes and maximize the products. If you have no idea of how much product you are going to get - or what ratio of reactants to use - there is no way you are going to be efficient in your processes.