People don't use them as bargaining chips most of the time--it is explicitly against policy. I'm sure it happens some times.

What they do do is send one when someone else does something nice (like fix a bug from a project they have left or whatever else). If you ever need something similar again, the person you peer bonused has warm fuzzies about the experience and a hint that they might get it again.

People also use peer bonuses during perf time to demonstrate that the work they are doing impacts other people enough for a somewhat uncommmon thank you.