Language Goal- Purpose
Why learn other languages?
This can be answered both personally and generally. First personally, my wife's native language is foreign and she knows mine and it's only fair that I try to become good at her language. Also, I live on and off in Poland and I just think it's rude to not at least try to learn the language of the country you live in. Nothing that should be illegal or anything but just plain rude not to give it a try.
The general reasons are when you learn a new language you gain a better respect for the details and complexities of everyday life. Things as simple as ordering a beer or buying something at a department store or even having awkward conversations with in-laws (all the more awkward when you know you sound like a retard...).
Another reason is that I feel like the best window into discovering another culture is through the language. Now this might not be a big deal to that kind of person that's socio-centric and only cares about his 'country' culture or his 'urban' culture. But it matters to me. The practical reason is because the best way to understand your own language and culture is by learning about other people's.
So that's the purpose and I think it would be difficult to truly consider yourself a polymath if you didn't have some skill in other languages. So there's that reason and also the fact that I just like it and it would be a hobby regardless. In the end it's all part of the journey.