you don't have to really use it in real world when learning another new language. (although that would be really nice and better motivated)

a lot of languages are similar, but there are something difference from each other. you learn another way of doing things or a better way to do things, and much more, which will benefit you in a long run. such as, I felt i first time get the idea of recursion when i did it in Lisp.

that's why a lot of people have 'learning another language each year' in their new year resolution.

not to mention, it's better not to stuck with one language and reluctant to learn new things, for career-wise at least.