Another reason would be to have an experience that is a basic part of our human heritage. Never speaking a foreign language would be kind of like never dancing. Sure it's not necessary, maybe it won't make you money and not everyone will get into it to the same degree, but do you really want to die without having at least explored life a bit more?
There's pretty decent evidence that languages have a big impact on how we think (weak spair-worf hypothesis). It's but one easy way to introspect.
Anyways, I don't know if it should be required, but it's probably more useful than calculus or english literature, both of which most people also don't use.
So much this! Not all languages are "equal" in their ways how they can express/define certain concepts, thus knowing more than one language helps you define better what's going on.
A thing I personally really enjoy is reading Wikipedia articles in different languages for a more "complete" picture of the topic because that works a little bit like triangulation: What one language might leave out another language might explain in more detail also highlighting different cultural influences and giving me the feeling of having a rather universal understanding of certain concepts, which doesn't hinge that much on merely memorizing terminology as terminology keeps on changing while the underlying principles don't.
That’s “Sapir–Whorf hypothesis”.
And then there's just the connections within families of languages. Learning Spanish (really relearning, high school originally) and Latin (college), connects me more easily to the rest of the Romance languages even if it is just their written and not spoken forms (I can "read" [0] Italian almost as well as Spanish/Latin at this point, but cannot speak it beyond a few phrases or listen to it beyond a few more).
[0] Really mentally translating. No fluency, but sufficient ability to parse and identify language features and roots to understand the gist of a text I'm provided.
e.g. seeing ESL people write online, they often phrase questions as 'why a thing is happening?'. It always looked a strange word order, but it's a transliteration of the word order in a question in some European languages. Seeing that, feeling that, makes me write that sometimes as well now because now it feels comprehensible, rather than wrong.
Or, how we can say Quick, Rapid, Fast. And we can say Quickly, and Rapidly - but not Fastly. Why not? I used to be a lot more fixated on a 'correct' spelling and word use, and I tend more towards 'I want to continue the pattern, even if it makes a non-dictionary word'.
And it really makes it clear what you gain and lose in translation - and how much of a disadvantage that puts non-native speakers, but also in the sense of things you can't translate perfectly would be lost in your one-language world. For example (and I'm probably going to screw this up) ĉu vi bondormis? translates very cleanly into English as did you sleep well?. But bonlegu! - how does that translate? Like bon voyage, but for reading a book? I wish you an enjoyable read!? enjoy! ? enjoy reading it! Even with one single compound-word there isn't a direct translation, so you have to face questions like 'which bit is important?' 'what is it that I'm trying to communicate?' 'what would a native English speaker say in this situation, is there a completely different idiom which applies?'. Worse and worse for longer and more complex sentences.
And that - 'what am I really trying to communicate when I say X in English?' - is also a fun question to face.
Small, casual things, but backing up the other comments saying that it's some fairly easy introspection back on yourself which is one thing that can come out of it.
A different language is more than just a different way of sharing information; it's a different way of interacting with ideas, and it reflects a different cultural history, and a diversity of cultural histories is fundamental to maintaining our heritage. When we lose languages (and we _do_ lose languages, all the time) we lose histories, traditions, and knowledge, and we also lose a specific and irreplaceable tool for looking at the world.
It also unlikely that we will ever cohere on a single spoken language, especially as tools like good neural-machine translation become much more powerful and more readily available.