You'll take my wristwatch off my cold, dead wrists.
I couldn't imagine sleeping without one. When I wake up in the middle of the night it's useful to know if it's 3am and I have lots of time to sleep or 6am and I don't. It's useful on weekends to know if it's 9am and I have lots of time to sleep or 12pm and I don't.
Being near blind a clock wouldn't work well, as I'd have to put my face damn near in front of it, and it would be a constant light source I don't want, anyway.
Plus, having the time on your wrist is very useful for discretely checking the time when in a meeting or whatever.
Lastly, they're probably the coolest thing you'll wear.