radnuker

Here's a couple. For me, not trashing your expensive leather band while working out. I have the modern buckle (quite an expensive band) and have no desire to sweat on it when going to Bikram Yoga but want the tracking info. I usually take the time to change to a white sport band but I'd much rather get an aluminum sport and not have to switch bands. 

Or, I don't want to ding my stainless watch when rock climbing so I'd rather have the aluminum on.

Also, battery life, so your watch didn't charge because it was slightly off-center on the charging base, grab your backup watch. Lame I know, but another reason to be able to pair with 2 watches.