Mole is a reference to the number of units of a compound or substance, without having to breakdown and account for every atom within the substance quantitatively. It is an easier way to do mathematical computations when calculating amounts of substances to react with amounts of other substances. It is calculated by using the atomic mass units of each atom in the compound's molecular formula. For example, a mole of glucose, which has the molecular formula of C6H12O6, would be calculated by adding together 6 carbons @ 12, plus 12 hydrogens @ 1, plus 6 oxygens @16. That would be 72 + 12 + 96, which would equal 180 atomic mass units. So one mole of glucose would equal 180 grams.
Another reason is that moles makes use of the unit gram, which technically is a mass unit. But the units we are talking about are much less than an actual gram of mass. It makes it easier to think about in terms of mass among the atomic levels.