But the front brake is normally controlled by the left brake lever, and signals are supposed to be given with the left hand. I've seen it argued that since most people are right-handed, and the front brake does most of the work and therefore deserves the most careful modulation, the usual practice is backwards: the right level should be for the front brake. I guess this is another reason. It makes sense to me, I guess, but not so much that I've actually bothered to swap the cables on my bike :-)

Anyway, in the wet it isn't always safe to use only the front brake.