Why is "Avogadro's number" used to count atoms and molecules?

Why is the $\text{dozen}$ a convenient unit to count eggses?
$\text{One mole}$ specifies $\text{Avogadro's number of particles}$. ${\text{Avogadro's number of }}^{12} C$ atoms has a mass of $12.00 \cdot g$ precisely. And thus if I know the mass of a reactant in a chemical reaction, I know reasonably precisely how many reactant particles there are, how many coreactant particles it will react with, and how many product particles I should expect to get after reaction.