Why does an aircraft need to be in balance?
When you walk, you are not in balance. You are always falling in one direction or the other, and you put your foot out to stop falling one way, and then start falling a different way.
If something stops you from placing your foot where you want, you fall over.
When you ride a bicycle, are you balancing?
No. You start falling one way, correct for it, and start falling a different way.
You are always making corrections, even if they are very small.
If the handlebars are suddenly locked in the straight-ahead direction, you will quickly fall over, proving that you were not balanced.
An aircraft just responds to the sum of the force vectors acting on it, and those forces are controlled by the pilot.
When a pilot turns so the wings are vertical, of course the plane is out of balance.
She is doing it in order to make a high-G turn or to turn downward.
She is planning her next move to end that maneuver and begin another.
Freeze the controls, and you quickly have a crash.
Here's Patty Wagstaff doing aerobatics.