Your head can still hit the dash if you have a seatbelt. It holds your body, not your head. And then there's things like side airbags, etc.

Supplementary is in that it supplements your main restraint - the seat belt.

They should always go off if the front of the car is damaged as the description here.

I think you may have gotten that wrong. Airbags are worse if the seatbelt is not worn. The timing for it firing takes into account that a seatbelt restrains the forward movement of the person. No seatbelt means moving forward quicker which means getting the full brunt of the airbag. Best case whiplash, otherwise a broken neck or similar.

At least that is what I have learned and which sounds reasonable.

I don't think your point and their point are mutually exclusive:

- no seatbelt, you want the airbag to catch you (better than no airbag)

- with a seatbelt, ideally the airbag doesn't (need to?) go off, but if it is necessary you'll be glad of it

I dimly seem to remember that there might be differing strategies in place in some parts of the world (maybe something like US vs. Europe?): In some places seatbelts aren't assumed in determining the airbag timings, whereas elsewhere the timing and triggering level are optimised based on the assumption that seatbelts are being used.

If there's blood, I think it's fair to say they should have gone off. I'd have those airbags checked and possibly replaced.

> They’re primarily intended to provide some protection for unbelted passengers

I don't think they're meant for unbelted passengers; they should be belted first. I once saw a program about the difference between regular airbags and airbags designed to be used without a seatbelt, and the latter were enormous monstrosities that could knock you out. They were determined to be a bad idea. (I wish I knew where I saw this, but it was decades ago and I don't.)