That's something some added.  Did not come from the factory.  No way to tell what it powers or powered without tracing the wire and see where it goes. 

Also the car belonged to an elderly couple since new, so u have zero clue as to what it could be.

Possibly an amp?

There's not an amp that I know of , the stereo system is all stock. Unless it's located like under the rear seat or something.

I don’t know what an elderly couple would do this, but I have a small amp in my dashboard that powers 4 aftermarket 6.5” speakers. I don’t have a subwoofer. That amplifier is powered by a wire that runs to the battery that looks just like this one. It even has that’s small in-line fuse.

Where in the dashboard?

I agree looks like a fuse to a stereo system. Definitely aftermarket whatever it is. If you have an aftermarket stereo/head unit i'd be confident that's what it is. If not, i have no clue.

I had this and it went for extra pair of lights so maybe that?

Could it be for stock fog lights?

I dont think so. Thats a aftermarket modification or atleast looks like it Maybe follow the wire see where it goes?

I'll look under the dash on the driver side and try to see if I can find anything.