I almost wrote a whole post on your question about why I learn languages (I learn them as an adult).
I like to remember what it is like to feel dumb. That may sound silly, but as a traditionally successful student, I forget what it is like to struggle.
After learning a language for a semester or two, I will travel to the country. In the past, I did immersion courses and stayed in a homestay situation (now I'm more likely to just explore). My brother's comment in China was golden. He said, "So this is what it feels like to be illiterate." I seriously considered volunteering to teach reading to illiterate adults - there are more around than we realise.
I learn to be resourceful, grappling with the words that I know to get the things that I need. My last time in Spain, I couldn't remember the word for 'fork'. Instead, I said 'The thing that you eat with'. It worked.
I learn about the kindness and generosity of others. By eating in local restaurants, I have to either decipher a menu or ask the waiter "What do you like?" The simple question has gotten me some of the best meals I've ever had.
I learn more about the ways others see the world. The phrases used in different languages helps me understand things like 'The nail sticking up gets the hammer', emphasising a culture's value of humility.
Learn languages. Throw yourself into new countries. You'll never be the same or think in the same way.