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.
I have spent a few years learning Spanish, and that’s something I use pretty much every day. I can understand it almost as well as English, but my speaking lags much further behind.
I would like to learn Mandarin, but unlike Spanish there is pretty much absolutely zero relation between English and Mandarin. And because I wouldn’t use it nearly as much unless I forced it, I’m sure I could spend over a decade studying it as hard as I have Spanish and be worse at it. So Mandarin is my answer.