Not everyone wants to just go with a 30-06! Or a 270, for that matter. A 300 Winchester Magnum doubles the range at which you can deliver 2000 ft-lbs of energy to a tough animal. It also gives you less bullet drop and drift.
Another example is the 7mm Remington Magnum. It shoots flatter than the 270, and hits harder than the 30-06. It out-performs both rounds in terms of trajectory and killing power. Ironically, the recoil from this round is about the same as from a typical 30-06 rifle.
The ultra-mags and Weatherby rounds give you still more killing power. They don't kill anything the normal mags won't, but they do increase the range at which you can deliver a large amount of energy.
As far as flat-shooting goes, the flatter the trajectory is, the less you have to ''hold over'' with the crosshairs.