The rotating drum tightens the paper as it rotates. The leading edge clip remains tight and the trailing edge clip is needed to take up the slack that naturally develops. The trailing edge clip needs to be spring loaded.
This doesn't answer the question of why they put the trailing edge on the right side. I wonder if it is possible to flip the drum around and move the awkward clip to the left side. This might require flipping the sandpaper template over and cutting your own paper with the diagonal going the other way (no pre-cut rolls).
On the other hand, loading paper gets easier after doing it about a dozen times.
Steve