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.
Dutch, French or Japanese.
Dutch because I would like to live in or at least visit the Netherlands some day,
French or Japanese because French is interesting and learning Japanese unlocks a whole other side of the internet (and UI design), and anime without subtitles.
Out of those I’d probably pick Japanese, as Dutch is relatively easier to learn by non-magical means.