A microwave has a fan to help keep its power components from overheating. Some units are combination convection ovens and microwaves and have fans in the oven cavity to circulate air when in convection mode.

 

No. There may be in some models a slow rotating gear reduction motor that turns a deflector (aluminum most likely) to redirect the microwaves in the oven chamber. There's also a turntable motor to rotate the food. The intention is to distribute the energy as evenly as possible. The fan is for cooling the transformer and magnetron. Other components may be inadvertently cooled by moving air as well. But the hottest parts are the transformer and the magnetron.

Though it seems to have been abandoned at one time(before turntables were universal( microwave overs called "stirrers"; Aluminum reflectors usually shaped like poorly designed four blade fans, that rotated about 60 RPM. They worked well to eliminate nulls but they also cost money so I think that's why we don't see them anymore.