Theoreticly because of the plate on the bone the cast is superfluous in most cases and is more to hold the joint in one position, had the brake have been not on a joint there would be little need of a cast at all.
Not really DK, the bones are still broken, the plate simply reinforces them, and helps prevent a break in future as the area will be slightly weaker for ages after the cast comes off, but eventually will be stronger than the rest of the limb (They way the bone heals, it grows over the area so is thicker in that place).
If you didn't have the cast, there would be a lot more freedom of movement, so the break wouldn't heal correctly as it could slip out of alignment. The ability to move it could also cause more damage as the bones can move in an unnatural direction, which could cause wear against the plate.
I know because I asked why we cast limbs at work even though they've got pins and plates in
Good luck with the recovery Greg
