Most modern microwave ovens without turntables are intended for commercial application and are known as commercial microwaves. A few very inexpensive models of residential microwave do not have turntables, however.
Commercial microwave ovens do not have turntables due to the use of a stirrer system in the microwave that moves the microwave source point around the food during cooking. While this serves the same purpose as a turntable in a residential microwave, stirrer systems result in more even cooking and avoid some of the issues of turntable systems. These include the need to clean the turntable separately and the possibility of the turntable being knocked off of the rotation system.