hemical equations usually do not come already balanced. Making sure they are balanced must be done before the equation can be used in any chemically meaningful way.
All chemical calculations you will see in other units must be done with a balanced equation.
A balanced equation has equal numbers of each type of atom on each side of the equation.
The Law of Conservation of Mass is the rationale for balancing a chemical equation. The law was discovered by Antoine Laurent Lavoisier (1743-94) and this is his formulation of it, translated into English in 1790 from the Traité élémentaire de Chimie (which was published in 1789):