lens - Why would I use a rear gelatin filter over a front filter?

Lenses such as the Canon EF 17-40mm f/4 L USM offer rear gelatin filter holders, but also accept front filters via a 77mm screw on type. If I was using a circular polarizer, I would need to put it on the front element to manipulate it. But if I was adding a solid ND filter or a color filter, what are the reasons to use or not to use the rear gelatin filter over the front screw on type?

This is the Canon EF 17-40mm f/4 L USM lens rear element. You can see the outline for the gelatin filter holder if you look closely. It is noted by the white square around the glass: