The car comes with 2 spots that will accept a bolt, just in case you strip out the threads where they bolt the negative battery cable from the factory (the one being used by the battery neg. cable in that pic).
I dunno why the previous owner had something bolted into the extra spot, but there's no purpose unless you have something else that needs to be grounded.

Best to just remove the cable from the battery terminal if you need to disconnect the battery, and if you do actually need to remove it where it grounds to the chassis, make sure you don't over-tighten when you put it back.