What Is the Darwinian Survival Value of Flying?

Quetzalcoatlus was a giant pterodactyl, the size of a Piper car or Cessna. It must have mostly glided but it probably tried to flap a bit. There is a suggestion that it might have punted itself into the air using its elbows and then got into a thermal. But it probably made use of thermals like modern vultures and eagles too.