99% of the time it does nothing. But that 1% it's a useful tool indeed.
Ah, that may be true with seatbelts, but not true with guns. There are 3 states in which a gun can exist:
1) Doing nothing
2) Being a useful tool
3) Being a danger to people you don't want it to be a danger to.
Seatbelts don't have a habit of accidentally killing people, or being so dangerous to possess that they're more likely to kill you or a member of your family than to be used successfully as a "useful tool" (in defense).