All chemical changes are accompanied by chemical reactions. These reactions are described in sentence form with the help of chemical equations in terms of symbols and formulae of the substance involved. Initially, the equation is in a skeletal form which is then balanced so that the total number of reactant species is the same as that of product species.

Why should a chemical equation be balanced?

Chemical equation:

1. A chemical reaction occurs when one or more substances, referred to as reactants, are changed into one or more distinct substances, referred to as products through the making and breaking of bonds.
2. To obey the Law of mass conversation(mass cannot be created or destroyed) or the Law of mass action(total mass of reactant and product species must be the same in a chemical reaction).
3. it is necessary to balance a reaction because the total mass of components before and after a chemical reaction should be identical.

Example:

$6{\mathrm{CO}}_2\left(\mathrm{g}\right)+6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\left(\mathrm{s}\right)+6{\mathrm{O}}_2\left(\mathrm{g}\right)$

Hence, to follow the Law of mass conversation and the Law of mass action a reaction is needed to be balanced.