It can be for a variety of reasons. The two most general reasons would be for:
1. A show of pilot precision (Airshows)
2. In a combat situation, to minimize radar appearence on an enemy scope. (You could have 3 aircraft in a group, but if they are flying in a close formation, it might look like only 1 aircraft on the enemie's radar.)
Trick airplanes fly in formations.
Because cats do not have the capability to fly airplanes.
Because people are not able to fly without a airplanes.
You are unable to fly airplanes, however you can fly helicopters.
airplanes can not but satellites can
Type your answer here... airplanes fly in the stratosphere to avoid weather problems
Some airplanes fly through hurricanes to get wind speeds etc.
Lift makes paper airplanes fly, just as it does real planes.magic
2,000 airplanes fly in the air each day.
They fly.
Airplanes fly, trains don't.
Airplanes fly because they cant swim.