It "locks" things into the track.
The cleats must turn 90* to come out so, in theory a end cap on each side could prevent theft.
This whole thing assumes no one would ever have channel locks, pliers, or the actual tool to loosen the end caps.
Other uses would be to keep things (such as a large box) from shifting in the bed.
Some have modified them to actually lock items in place in the bed.
