Putting in a little bit of time to raise the fondness of the girlfriends can unlock their "special abilities": these can help in particularly difficult missions late in the game.
Reaching 100% fondness with all five girlfriends is not required to achieve 100% completion of the game.
Unlike the male friends, however, dating is more than just worrying about the venue.
To have a successful date Niko must cater to the women's tastes in three areas; clothing, vehicle, and how fancy the date is.
Dating these girls is not a requirement to attain the 100% game completion stat, however each has a special ability that is unlocked once they like you enough.
This feature is available after the “Out of the Closet” mission in Brucie's mission line.