It's a bit hard to tell from the picture, but it looks like there are two openings in the connector. Is that right? This would argue against its being a ground and in favor of an indicator light of some sort, or a simple switch. And Subaru switches are seldom simple ones.
If the style of the cable and the connector match what's already there, I'd say it's intended for some accessory that didn't come standard with your model. It's much easier and cheaper for them to use one standard wiring harness that's configured for every possible thing that might be connected than to tailor them to the fine details of different configurations.
If the cable and connector are of a significantly different style than those already there, then I'd guess it's left over from an aftermarket device once installed by a previous owner.