Reheating, popping popcorn, and occasionally defrosting.
Early on I also use to use it for making bake potatoes but I seldom eat those anymore. I used it for making bacon a few times in the past.
The idea of microwaves being used to make meals from scratch failed for the most part. Most people still prefer to use their conventional oven to bake, broil, and roast foods.
The microwave will never (replace) the traditional oven.
Every now an then I see some old microwave cook books on the bargain table at bookstores. :-)