A microwave works by bouncing microwaves around the interior. Since the shape of the container doesn’t change neither will the path that the bounced waves take. This can lead to hotspots in what you’re reheating.

To mitigate this you have a few options:

• move the food around in the container so that different parts pass through different hotspots over time (this is what a tray does)
• interrupt the microwave path via a “stirrer fan” that sits below the microwave floor (this is what tray-less units use)

Both approaches redistribute the hotspots to maximize even heating. The efficacy of either approach will come down to the specific design of either unit, but a tray-less unit can be easier to clean, and with fewer moving parts exposed to end users can be a good option for commercial/high user count settings. Each design accomplishes the same task.