My swim coach had me doing free and back. I never asked why back when she knew I was training for triathlon. I think it may have something to do with exercising the opposite groups of muscles. For me, back blows out my thighs meaning I'd be completely toasted for the ride so it's not even good as a fall-off from free in an open swim, etc. I did it when she requested it, but I never practiced it during lap swims (I can't hold a straight line and need a lane all to myself if I want to practice it).

Since I've stopped with my swim lessons I feel I need something else pushing me on. Masters may be a good thing, or maybe I'll look for another coach.