Why high level people on Rocket League rotate with their cars while they are flying? Do they fly faster or higher or what?
Your speed is identical, but spinning gives you the potential to:
- make slightly harder hits by using torque to add power the car's contact with the ball (this is the reason that flips result in harder hits than simply driving at a ball),
- make the resulting hit a bit tougher for the opponent to read, as the direction of your spin combined with your boost allows slight adjustments in midair,
- practice better midair car control since even a live match is still an opportunity to improve your skills, and/or
- make the resulting replay just a bit flashier, as spinning doesn't make the car any harder to control when you're used to it.
In addition to what @Koviko mentioned, another reason is for better control. During eg. a tornado spin, as the bottom of your car spins quickly from pointing to the lower-left to the lower-right and back again, you can make micro-adjustments to adjust your car's path in the air much more quickly than you could if you only rotated your car along a single axis.