This is especially important for casseroles that have a decent amount of liquid or gooey cheese in them. Juices from meat, milk, and broth bubble around, and when it's piping hot, those juices are super runny. If you cut straight in to that lasagna, you may get a soupy slice and a lot of dripping.
When we let our casseroles rest for 10 to 15 minutes, we find that the liquid gets reabsorbed a bit (our highly scientific observation), and the servings come out cleaner.
Another reason: Casseroles are hot. They bake for a long time and hold a lot of heat. Letting them hang out for a few minutes allows them to simmer down to an edible temperature. Don't worry that resting a casserole will mean it won't be hot when you plate it. It will be.
Related: Recipe: Chicken Divan Casserole, from Scratch (pictured above)
(Image: Nealey Dozier)