Why is the mole an important measurement in chemistry?
Because it is the link between the micro world of atoms and molecules to the macro world of grams and kilograms.
The mole allows us to establish the idea of equivalent weight. Given a mass with a definite chemical formula, we can calculate precisely how many molecules or particles there are present in that mass.
For a simple chemical reaction that yields 18 g of water, we thus know precisely (i) the number of water molecules involved; (ii) that this is the same number of oxygen atoms involved; and (iii) the actual masses of the species involved in the reaction. This treatment leads itself to stoichiometry, and mass equivalence. In addition for chemicals whose mass is difficult to measure, such as gases, we have the tools to determine just how many molecules are present in a given quantity of gas by the various gas equations.
So given a mole of