Some raptors eat on the wing from time to time or quite frequently; Peregrines will eat while on migration as they cross large bodies of water. Kestrels and Merlins eat butterflies and dragonflies while in flight, and kites will eat insects on the wing daily. But, you don't see buteos eating on the wing very often at all....except when they are wintering in an area with a large concentration of raptors. The competition between birds is fierce at times, stealing prey is easier than finding food on their own. Normally buteos catch a mouse or vole, and eat it on the ground, or take it to a perch.
But, at places like Lehi or Snowville, where there are numerous buteos in one spot at certain times, if a bird catches prey and stays on the ground, it is surely to be harassed and forced to give its prey up.
So, many birds eat on the wing to avoid being swooped upon, like the adult Harlan's Red-tail pictured above.