I don't think it's a want as much as a possible convenience. I have the Garmin Vivofit which has a year long battery and is waterproof. I never, ever, ever take it off. Because of that I'll never accidently leave it at home like I do my work badge, smartphone, etc.
It's just one more thing to forget... that's the only reason, I think.
Exactly! LOL Especially since I seem to always be running late in the mornings and am always in a hurry!