1. Making Friends
This is pretty obvious. If I got a dollar for every time a new acquaintance said "Oh phew I'm glad you speak Japanese because I'm no good at English" I'd have like $17. Speaking Japanese allows me to communicate with a wider variety of people--including little kids, immigrants to Japan from other countries, old folks in the countryside, etc. After I left university, the number of people I met who could speak English dropped dramatically. Speaking Japanese can also help you avoid the kinds of "friends" who just want to practice their English and aren't really interested in you as a person--you're not dependent on them for your social life.
Also, as an exchange student, I lived in a dorm with Chinese, Korean, Taiwanese, Malaysian, Finnish, British, New Zealander (?), German, French, Polish, and Dutch students. The one language we ALL had in common was Japanese. Studying Japanese together allowed us to connect even though we had very little else in common.
Also pretty obvious. My first job in Japan, the company provided me with everything: an apartment, a bank account, a cell phone, all the paperwork in English translation, even a guidebook on the culture. They were clearly ready to support the fresh-off-the-boat types. It was very helpful, but can you imagine having to go to the doctor with YOUR BOSS because you can't speak Japanese? Nope. But that kind of dependency was part of the company's strategy, I think. I knew if I didn't like the job, I could quit and find a new job and apartment by myself. For my coworkers who had never been in Japan before, that option was mind-boggling to even consider, leading to some sticking out disappointing situations because they felt they had no other choice.
Romantically as well, I felt confident I could come to Japan and date my Japanese boyfriend (now husband) without any awkwardness of a dependent situation when we hadn't yet committed to marriage. My boyfriend was very supportive in a lot of ways, but he never had to hold my hand as I took baby-steps in Japanese life. I wouldn't have liked that and I'm glad I passed that stage before I met him.
3. Discoveries Off the Beaten Path
There are at least two stories I can think of that are good examples. One was when I traveled to Okinawa with a friend. We had gone to the famous Shuri Castle, and were wandering around some little narrow streets in the area, one of which went through a neighborhood. As we walked along it, we came upon a rain-smeared handwritten paper sign that said "300-year-old Akagi Tree this way" in Japanese. So we followed the tiny overgrown path and discovered a magical place--the largest tree I'd ever seen, sheltered by a huge vine-covered volcanic cliff, and an Okinawan-style shrine. After looking up the trees, I found they had once grown all over the Naha area, but were largely destroyed in WWII. I don't think we would have followed that path and discovered the trees if we couldn't read Japanese. We wouldn't have known what it was about at all!
The other time Japanese allowed a rich experience was when I went to Nikko by myself. All the information about local onsen (hot springs baths) was in Japanese. And I love onsen, so I visited many small ones that turned out to be amazing, using information that was meant for Japanese tourists and not foreign ones.
4. Job Opportunities
Although I haven't personally taken advantage of this yet, I do know foreigners in Japan can get more job opportunities (i.e. other than eikaiwa!) if they're fluent Japanese speakers. Of course you usually need another marketable skill or two, like accounting or sales or translation or tech-y experience, but two languages definitely make you more attractive to most international companies than one!
I'm hoping my skills will also one day open doors for me back in the U.S.--I love Japanese and I enjoy teaching, so if I can speak Japanese well, I might just be able to enter a niche with less competition than if I only spoke English.
This one might be less interesting than the others but for me it's so worth it! Now that I'm all married and wife-y I cook. Pretty much every day! I'm not a natural at cooking and need to follow a recipe carefully for stuff to taste good. My husband is Japanese and though he loves Western dishes, I enjoy being able to cook us a big pot of miso soup, or a quick oyako-don dish, or a roasted mackerel. You can find a lot of those recipes in English too, but I like using a online service called CookPad--all the recipes are uploaded by users, nearly all in Japanese, so I can find authentic recipes for the homiest Japanese comfort foods. I think being able to make Japanese dishes will be a great omiyage (souvenir) to my family should we one day move to America--I can help keep my husband's Japanese heritage alive in our family.
How about you? If you've ever studied a foreign language, what were the benefits? If you've been to Japan, what was your experience with/without speaking Japanese?