Proficiency in reading and writing is clearly useful.

Proficiency in musical instrumemts is not necessarily useful by itself, but it's a path towards mastery of something hard that typically involves a lot of intentional practice. Having been able to master something hard once makes it easier to master other hard things, because you can reuse the learning skills. Music has a little bit of applied fractions and what not, too.

The same thing learning to learn journey happens with learning a second language. But also, learning a second language helps develop skills and understanding of your first language, and helps develop communication skills for situations where there's a limited mutual understandable vocabulary.

Some languages have market value too. Better communication with employers, customers, and suppliers is valuable.