Like Doc said, those just keep the boat from bouncing on the bunks.
However, looking at that stitching, I wouldn't use that tie down on anything I cared really about. It looks like the overlap wasn't lined up, and the stitching isn't completely holding the two halves together, it's kind of a mess. Drunken Chinaman at beer:30 put that one together.
You'll probably be fine with it, but just fyi.