Rollfilm backs open up a variety of options. Don't have a long enough lens, but you want to make a lot of exposures?--Rollfilm can be a good option. Or say you're shooting handheld press camera style with a 4x5" rangefinder camera but don't really need 4x5" and you want to have more exposures available without having to deal with filmholders. A rollfilm back and rangefinder can also be a good option for portrait sessions. Or maybe you're traveling and don't always need 4x5", but you don't want to carry two different camera systems.
There are also good reasons just to shoot 4x5" all the time and crop. Sheet film can have better flatness than rollfilm. There's something to be said for having only one size negative to file.
It all depends on what you're shooting, and what your work habits are.