Answer
To avoid the turbulence and power requirement is less for those high speed at such a high level.
Work Step by Step
The supersonic airplanes need to fly at a high altitude because of a simple reason that the higher we go in the atmosphere, the air is less dense and we are able to fly at greater speeds as there is less air resistance so they achieve the good speed with less power and no supersonic sounds near the surface and thus there they also pretty much avoid the turbulence.
