Answer: To test it for spaceflight.
Most space-bound rockets launch from a static position on a pad on the ground, pointing straight up into the sky. Naturally, they will also be tested in a similar fashion, which likely doesn’t garner much attention. That changes, however, if you’re like Virgin Orbit and decide to launch your rockets by dropping them.
On Wednesday morning in Southern California, the company tested its relatively new rocket — LauncherOne, built for carrying satellites into orbit — to see if it would perform as expected at launch. Instead of a vertical takeoff, LauncherOne gets to space by being dropped from underneath the wing of a plane at high altitude, at which point it ignites its engine and flies up.
Wednesday’s test did not require the rocket to fire up its engines — instead, the focus was on completing a successful drop from the plane in order to gather data on what to expect in a real-live launch. The Virgin Orbit team wanted to know exactly how the drop would go and if the rocket could withstand the pressures of the fall before its engine ignites.
Since the engine was not ignited this time, the test concluded with LauncherOne crashing to the ground at Edwards Air Force Base, where the team will recover what is left. Presumably they have more rockets, though.