Both Enterprise and Voyager are flown up to a height of about 50,000 feet (46,000 in this test). The advantage of launching from higher in the atmosphere isn’t gravity, but in air resistance and inherited delta-v. Launching from a standstill at ground level, into much thicker atmosphere, requires a great deal more propellant. In this test, the Enterprise fired her rockets in a 20 second burn — considerably less than the 70 seconds required for a full space flight, but longer and higher than she’s been before.
The purpose of this test was to check the performance of the aircraft’s feathering mechanism. The official chart below is a bit hard to make out — someone needs to talk to Virgin Galactic’s graphics designer about that — but the full version is a bit clearer if you click on it. The ship’s wings launch in one position for the rocket boost, but then rise to help the craft brake for orbital reentry. This was the first time we’d seen the mechanism in action at this speed, but by all accounts it worked flawlessly.