When: Whenever I legally can.
Why: Due to Mom's bad taste in men, I learned at an early age that you can't depend on the police to keep you safe. This lesson was re-inforced later in life on several occasions.
I eventually had situation awareness beat into me. Thankfully haven't been involved in a fistfight in a few years, and I've never cleared leather against a two-legged critter.
Personally, I dont see why someone only wear their seatbelts part time. I dont know of anyone who buys insurance when they only go through high-traffic, accident-prone areas.
An automobile accident can happen anywhere. So can a crime. If the seatbelt works, I'm wearing it. If I can legally have an effective tool for self-defense within arm's reach, I'm going to.