Why is the mole an important unit to chemists?

is important because it allows chemists to work with the subatomic world with macro world units and amounts.

Atoms, molecules and formula units are very small and very difficult to work with usually. However, the mole allows a chemist to work with amounts large enough to use.

A mole of something represents ##6.022x10^(23)## items. Whether it be atom, molecule or formula unit.

Defining the mole in this way allows you change grams to moles or moles to particles. Even though you can't see the particles.