It isn't absolutely necessary that a chemical equation be balanced; however, if one wants to do any calculations (stoichiometry, percent yield, volume of a gas produced, solubility product problems, equilibrium constant problems, etc) then it is necessary that an equation be balanced. Trying to work a problem in which the number of atoms we start with is different from the number of atoms we end up with is not logical; therefore, we balance the equation so that we have equal numbers of each atom on both sides of the equation.
