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:
Gels aren't generally available as screw-on type filters. To place a gel on the front of the lens requires a holder of some kind and a much larger quantity of the filter media. Many "sample" sets of various color gels come in sizes that allow for trimming to fit a rear filter holder but not large enough to fill a front sized filter holder. Due to the less durable materials a gel filter is made of, it is protected from potential damage when inserted in a rear holder.