Why, benefit for our plain and simple Garak, of course. The Human needs help; Garak will provide it. Garak can be his confidante, his guide to Cardassia. Garak can empathize with his harsh treatment, oh, so very unfair, at the hands of his Federation masters. Garak can pump all the useful information out of him, and then bring him to Tain to be sacrificed on the altar of Garak’s ambition.

Elim Garak, before exile, is not a nice person. (And, debatably, after exile he isn’t much nicer.)