Question

why must we balance all equations?

Answer 1

Because of something called the Law of Conservation of Mass. This law says that for a particular system (e.g. a chemical reaction) where neither matter nor energy can enter or leave, the total mass within the system must remain constant.

In other words, since you haven't added or removed anything from the system, whatever mass you end up with needs to be the same as what you started with, even if it's in a different form (e.g. products that are different from reactants).

This is why balancing is important - it ensures that for whatever mass you started with in the reactants, you're accounting for in the products.