As long as one uses a replacement glass tray with the same configuration as the original, putting a new tray in the microwave is just like removing, cleaning and replacing the old one. Having a tray with the same diameter isn't enough, as the mounting slots must stay the same.
People who are looking for a cheap deal on a replacement glass tray for a microwave may end up with one that has different groove lines. When they put this into the microwave, the new tray does not rest flush against the rotating ring. Simply replacing a 12-inch microwave glass tray with another tray with a 12-inch diameter does not mean that the new tray fits. The replacement tray does not have to come from the same manufacturer as the original, as aftermarket parts are frequently available, but it does have to fit the same way.
The rotating microwave glass tray helps food cook evenly, as different spots in the microwave conduct heat differently. It is important to remove and clean the tray on a regular basis, as residue from food can collect, potentially forming a health hazard. Without a spinning tray, the food is likely to cook unevenly, leading to undesirable results.