I vote for vibration damping and to protect the side of the tank from wear and possible leak.
A couple years ago I flushed my tank as part of winter maintenance and a plastic piece fell out. After some online research I found out that it goes between the fuel pump mount and the side of the tank. It's a flattened "U" shape that just slips on the edge of the sheet metal mount edge. There are two of them. Oh yeah, one of them is a pain to get on too. My wife had to do it, my hand was too big.
Rod