I thought I knew what they were used for when fitting seatbelts/harnesses but I have confused myself, so before I complicate matters, what are they for, and do you have to use them?

Use as a spacer to raise the seat belt fitting away from the car to allow room for the fitting to swivel so that the belt isn't strained pulling at an angle , normally has a wavy washer as an anti rattle device

That's what I thought, so I just acquired some spacers from an mx5 thinking these would be ideal as would take 7/16 thread only to find on both sets of harnesses i have the hole for the bolts are 7/16 so won't fit over a spacer to allow the movement. One set is an unknown 'cheap' harness and the other trs. The size of the holes suggest needed to use a smaller than 7/16 bolt, which doesn't seem right. Can only assume manufacturer intended them to be fitted without spacers. Not sure which idea I like less...