Why people rotate during flight in Rocket League?

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.