I wanted to learn Japanese because of my aspiration as a game designer. You know, since most of the best games ARE from Japanese developers... IMO. I also wanted to learn it 'cause the pronunciation is very similar to Spanish (which I also speak).
But that was back when I was 12. Now I'm majoring in Computer Science, uninterested in making video games for the time being. Either way, I have learned too much Japanese for me to stop studying. Also, I'm a language lover. Comparing the various aspects of English, Spanish, and Japanese is hella interesting to me.
I plan to live in Japan in the future, at least temporarily. Maybe I'll land a job there as a programmer if I work hard enough on my Japanese.
IMO, Japanese is useful if you're interested in anime, video games, manga, or you're actually visiting Japan. Or you do a lot of business with Japanese companies...