The practice of directly basing the appearance of a character on a real person is sometimes done to make it easier for artists to animate realistic emotions on the character's face.
Valve based all the major characters in Half-Life 2 off of real people for this reason, though they were simply models chosen as a visual aid; for example the real person that Dr. Eli Vance was modelled after was a homeless man found near the Valve offices.
The other major reason this is done is when a character is voiced by a "name", in order to make the character resemble the celebrity who provides their voice. This extends as far back as 2003's The Chronicles of Riddick: Escape From Butcher Bay, where the antihero Riddick was modelled after actor Vin Diesel. More recently, the practice has been seen in Halo 3: ODST, in which Sergeant Edward Buck and Captain Veronica Dare were modelled after (and voiced by) actors Nathan Fillion and Tricia Helfer, respectively.