Butterflys move from a larva (caterpiller) to a pupa - the hard brown shell hanging upside down from leaves.
The stage when they are that hard brown shell they are transforming into a beautiful graceful butterfly.
From within the pupa they are confronted by the hardship of getting out of the pupa.
It is really hard and a genuine birth.

The extreme stress of struggling out of the shell forces blood into their lifeless wings.
Once out of the shell they dry, recover, open their blood filled wings and fly.
Without the confrontation of the shell and the stress endured while getting out they would not have blood forced into their wings. They would die.

The stress enables them to fly and they to be butterflys.