I recently replaced my rear brake pads and, while doing so, a couple odd looking washers fell out. At the time there didn't seem to be anyplace where they belonged so I set them aside and put the car back together. However, when I got curious and started looking on the internet I came across a video that cautioned me to be sure and put them back but didn't tell me where. I then searched until I found where they go (between the caliper mounting bracket and the knuckle with a slight crown the sits inside the hole in the knuckle.) and now I would like to know what they are for and if I really need them. I don't mind doing the work if it's necessary but I'd like to know why. Oh, and by the way, I lost on so I'll have to either find it (unlikely) or buy one or two.
I appreciate any advice you can give.

 

Not 100% sure why. It likely centers the caliper bracket to the rotor, prevents corrosion between the bracket and hub. Maybe dampens vibration?

I'd install them when you have time, but probably not catastrophic if they aren't installed.