I know a bit of Spanish because I holiday in Spain occasionally, although lately it has become valuable as we are working in LatAm a bit.
I know passable Italian purely because it was enjoyable to learn. It has since been useful on two vacations across Italy.
And finally I'm learning Polish due to my partner being Polish. It's hard as hell compared to Romance languages, but it'll be worth it when I can have a conversation with her parents beyond courtesies.
On the flipside, it made my Spanish vocab quite a bit worse as I keep recalling the Italian word in place. Happily this doesn't lead to many problems in reality, most Spaniards I have interacted with have either understood the word anyway or I could recall the correct word quickly.
Are you interested in Japanese culture, history, literature, religion, or people?
Do you or your company want to do business in Japan, with Japanese companies, or Japanese people?
Do you have friends, family, or a significant other who's Japanese? Do you want to make new Japanese friends or contacts?
Are you interested in what the Japanese have to say, and want to read or hear it in their own language?
Have you ever wanted to visit Japan and feel less like a tourist and more like a native, to be able to better know what's going on around you, read the signs, maybe have some ability to find your own way?
Well, if you're not interested in any of these things, perhaps you'll forgive or at least understand some others if they actually are interested and find these things valuable.
One of the things that exists in their culture that doesn't exist in the US is the notion of "oneness." If you get on the train in the morning and observe people going to their jobs, you'll notice that there's almost a "uniform" - very similar suit, haircut, and overall style. In America, individuality has become a disease that has lead to everyone feeling the need to be special and different. I'd argue that this 'disease' is the plague of our politics and economy, as companies nor politicians typically think about society's greater good ( save a handful who are now trying to align business with humanitarian efforts ).
Also, embedded in the language is respect. You use different words and sentence structures to address your friends vs. your boss.
So, what is the value? Probably experiencing life in a different way, and gaining more perspective.
tldr: it opens doors that would not normally be open to you
Note that this is valid for more countries than not: learn the language, and the locals will look on you more favourably. It is still a must to work well in places like France and Italy, for example (albeit less so than 10 years ago).
English is a fallback language, but the preferred option is still the native lingo.
It lets you experience things you could not have otherwise.