We cannot see atoms, molecules, or formula units with our eyes because they are too tiny. In order to create matter which we can see, there are millions of billions of atoms. Because there are so many atoms/molecules/formula units in matter, scientists grouped them into mols so it is easier to calculate...just like we buy eggs in dozens instead of individual eggs or paper in reams instead of individual pieces (imagine how long it would take for the school to buy paper if they had to count and scan each individual sheet).