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.
If an Internet girlfriend's fondness is raised to a high enough level, her "special ability" is unlocked.
Like in GTA San Andreas, two of the girlfriends are introduced during the storyline.
Conversely though, Niko's choice of wardrobe and vehicular transport will also affect whether or not his date has a good time.
Both Michelle and Kate like Niko for who he is, so wearing something upscale (i.e., anything not from the Russian clothes store) and showing up in a flashy ride is typically to be avoided when dealing with either of them.
The date and time of the first date will be added to the organizer in Niko's phone.
The first date is very important as it will determine whether or not the girlfriend will give Niko her phone number so he can continue dating her.