This would be great if you wanted to talk to yourself without anyone understanding what you’re saying, for example, or if you didn’t want anybody to know what you were writing down. It might also be useful if you enjoy lots of foreign language media (see music, tv shows, movies) and want to experience the “original” rather than through dubs and subs.
Personally, I would go for either Russian or Estonian, both of which are because I frequently listen to lots of music that are in those languages, and they aren’t spoken by any of my friends.
My son-in-law is learning Finnish. That’s on top of the English and German (Army brat) he already knows. As for me, I know Spanish and some German, and while Finnish would certainly qualify as a “nobody speaks” candidate, I’d go for Dutch. I’ve been to the Netherlands a couple of times, and it seems like a fun one to learn. There’s no Dutch community locally that I’m aware of.