What's the point/benefit of roasting a turkey on top of a bed of vegetables?

If you cook the turkey on a rack, you can get good color on the skin of the underside. It may also roast slightly faster, since there is air circulation all around it. Roasting veggies as supports also helps, but cuts down on the circulation so the skin won't brown as much. But it sure makes for yummy veggies to eat with the meat, and adds flavor to the gravy drippings that will form. Pick dense veggies, though, so they don't burn before the turkey is cooked. Stick with things like potatos and thick slices of carrot, turnips, etc.

Kristina Bifani -
Some people like using the vegetables to get a "two for the price of one" effect.
The vegetables support the turkey off the bottom of the pan, as a rack would, to prevent burning and sticking.
They also exchange flavors with the turkey as the whole pan roasts.
Then, some serve all the vegetables in a separate dish. And others use some of the vegetables when making the gravy.
Your choice - whichever you like better.