Ideas:

(I'd be more interested in controlling my phone from the wrist)

Two-factor auth codes on the wrist instead of unlocking phone.

Beep my phone button (so I can find it when it's lost).

Record conversation. I have bad memory in long meetings.

Pager. A simple 'push this button to buzz the other person who is also wearing a band that you are linked to'. My wife needs me and sometimes I have headphones and can't hear her and don't see the phone light up.