Sounds to me like they may have used it for a RV?.
I wondered about that - especially since it is right next to the door on the wall that is only 2 feet wide or so, and easy to run outside.
First thought was a charging station for an electric car, but the house is 15 years old - too old for that. The lady that lived ther had not a clue.
It would have to be a huge RV, think motorhome with dual AC units. Normal RV plugs are the 30 amp 120 volt type with 3 prongs.
I know someone here can tell me what it would be used for
I think it is intended to plug a generator into the house wiring.

I think it is intended to plug a generator into the house wiring.
I don't know RVs specifically, but I think they mirror what we use in the marine world and that's definitely not a standard 50A/250v connector for us.
Typically used for.....
In a garage, backfeeding the panel with a suicide cord. Or an oven/dryer. It is not a standard welding plug.

Backfeeding is a merely a "Bonus"
You ever see a Miller Trailblazer or other Welder/Gen? Most have 14-50, and thusly a lot of portable type welders go with the 14-50. All 4 of my welder outlets are 14-50.
It a 240 outlet, code requires a neutral and a ground with the two hot wires. Check your panel to see what 240 breakers you have. Shouldn't take to long to track it down.
Many of the newer RVs are coming with the 50 amp service, even if not equipped with 2nd air or washer/dryer.
My welder plugs are all the standard 6-50 three prongers but I did run 6/3 so if I ever get a fancier wire feed that needs that 14-50 I can swap the plug in.
My Maxstar has a 6-50 on it...if it weren't a factory plug (Miller MVP) I woulda put a 14-50 on it long time ago.
I too ran 6-3 to all my welder outlets (Ran 8-3 to the compressor shut off) THOUGHT about putting a 6-50 on one of them but then I figured, I ran the 6-3, may as well use it, PLUS my house garage has had a 14-50 all along for the generator reason. I've already got a handful of adaptor cords.
I will say there's a been a few times where I liked having 50A-110v.