in a fit of desperation I tried it once, it worked surprisingly well
I got curious, googled, and found this:
… mirin actually is a sweet rice wine used in Japanese cooking. It doesn't just flavor food. The sweetness also gives luster to sauces and glazes and can help them cling to food.
If you don't have mirin, there are many common suggestions for substitutions. You can just use dry sherry or sweet marsala, for instance. Or you can dissolve a small amount of sugar in a little white wine or sherry, perhaps a 1/4 teaspoon of sugar to 1/4 cup wine wine.
and now it makes sense.
plus, mirin and maple syrup alliterate, so there's that too.