Its for when you remove your soft top, it keeps it closed. The soft stretchy elastic loops with snaps are for rolling up the back window, not the hard plastic tabs. There is a second set of snaps that you can snap those down to keep them from hanging down while the top is in use.
this post explains it best-
Soft Top HELP!!!
Originally Posted by Nuke83
Those straps hold the frame in place with the spring assist when the top is uninstalled. When the top is installed (after installing) on your Jeep, you release and leave then unhooked. When you're preparing to remove the soft top, you need to rehook these in the original position. If you remove the top without hooking them, the spring assist will make it quite difficult to keep the frame neatly folded while off the Jeep. The straps keep the spring assist compressed when the weight of the top isn't doing so.
Whatever you do, don't cut them off the Jeep. When new, there's a paper hang tag warning you about this.
Also, the snap is to slide them out of the way and snap at the top of the bow when the top is installed. Prevents you from having the strap from flapping around or getting damaged if pinched in the folding frame or damaging the frame if getting pinched.
Here's a bit more on them:
2013 Spring assist Soft top storage..those straps!