Here's the reason I use one, at least for my home defense handgun. I had a back injury last January, thus, I cannot walk steadily without my cane. This precludes me operating a gun with one hand, and a light with the other, since my Glock 17 is in one hand, and the cane is in the other.
So, I HAVE to have a weapon with a rail.
My experiences may be different than yours.