Why launch an orbital rocket from 35,000 feet? What will that do to assist the rocket to achieve orbit? Orbital ability depends on speed, not on height above

Why launch an orbital rocket from 35,000 feet? What will that do to assist the rocket to achieve orbit? Orbital ability depends on speed, not on height above the ground. They'd be much better off launching a larger rocket (more fuel = more energy) from the ground rather than a smaller rocket (its size limited by the lifting ability of the plane) from 7 miles up.