If I recall correctly that plug was previously found by a member searching for a permanent live connection in the post that showed me where to get power for my rear, internal, boot, LEDs. It is located tucked in behind the rear fuse panel on the left side of your boot.
Indeed it appears to be sitting there as a totally unconnected plug. Very similar in concept to the many additional plugs found within a domestic PC that are for future use.
Jeepatron has suggested a great idea for it's use
Keep in mind that it can be used for anything you need it to be. Simply cut off the plug, add a spade connector (or a heat fused/shrink connector) to your new toy and off you go
I may have my LEDs boot/cargo lights connected to it. Search for, "I have seen the light" as that was the topics name. I think