Vents on a microwave are designed to protect the microwave from getting too hot, and they are used on units that sit above an oven as well as ones that sit on a countertop. Vents and exhaust fans serve the same purpose.
The vents on a microwave that sits directly on a countertop allow for heat to escape versus becoming trapped between the microwave unit and the counter and causing possible damage to both. Microwaves that sit above an oven have an exhaust fan for the same purpose ? to protect the microwave from becoming too hot. The exhaust fan will protect the base from heat created from the oven or stove top below.