In the original function they were used for the number on the nose (I guess you knew that already). In the sportscar carshape they have no function anymore (because the numbers on the front of those cars are always placed on different spaces).
There exist Le Mans carsets from 1998 and 1999 by Chris Rimmer. They are very incomplete, but they include a good Toyota, Audi, CLK(R), Panoz etc.. And he also made the McLaren (available with a carshape). You can download those files from gprix2.com. Or you can PM me your email adress and I'll send them to you.
Here is my attempt on the Nissan (from December 2006, one of my first tries with sportscars, looks terrible):

In comparison the Maserati MC12, which doesn't look that rough anymore.

I have a lot of unfinished cars on my HD, basically because I always want to know if it's possible to make them for GP2.