Many recipes call for the nutty flavour of butter that's been slightly burned, often called brown or browned butter. You can make it on the stovetop, but the microwave takes less time and has less risk of overcooking.
The Maillard reaction intensifies the flavour of the butter so browning is an important step in many recipes. Cover the butter with wax paper or a microwave safe plate and cook it between three-five minutes. You're less likely to burn the butter and you don't have to wait for the pan to heat up. Check out the video above from CHOW for more info.