When your relationship level gets to between 35-50% a vehicle reward is received, while reaching 100% will earn you a new outfit, and other benefits such as free vehicle repairs or not losing your weapons when wasted or busted can also be conferred depending on your girlfriend's profession. Dating GTA San Andreas girlfriends does not count towards 100% completion, so other than the interactions required to progress the story you don't actually need to engage in these relationships, however some do provide useful rewards for progression.
