Quake BSP files

Hi to all. I''m traying to make a program that read an IBSP file from Quake 3 and render the map. That files store the geometric data in a list of faces. There are three types of faces, one of them are Polygons, that are a list of vertexes. I''ve noticed that the most polygons are triangles, but they are composed by six (or more) vertex that must be rendered like a triangle fan (sharing the first vertex of the list). My question is : What is the reason to store the triangles in that way? Why it is better than using only tree vertexes and render them like a triangle list (of one primitive)? Maybe the actual 3d graphics cards are faster with the triangle fan, but I''m not sure. Is that correct? "If you''''re gonna die, die with your boots on"