Vasoconstrictor. Naphazoline is a vasoconstrictor. It's purpose is to constrict the blood vessels on the white part of the eye thereby making the eye less red. While it is effective for short term relief, over use of this drug can actually lead to prolonged redness.
Yes. These drugs can supplement each other. Any systemic effect of the eyedrops is trivial compared to the strength of the Benadryl (diphenhydramine) so the combination is useful if the allergic symptoms are affecting other parts of your body.