In June birds are in full abundance. They are on the ground, in the trees and flying here and there. But why in the air? Why do birds fly at all? One good reason is that they escape ground-dwelling predators. Another is that the air is filled with nutritious insects. Here is a whole new environment available to whomever can enter it. Over time any slight hereditary modification in a bird ancestor, which at first enables soaring from a tree or a brief taleoff while running, when further enhanced leads to flight.
What about insects, most of which fly? What advantages does an aerial capability confer upon them? One great advantage is faster locomotion. It would take a fly a long time to walk from Saratoga Springs to Ballston Spa, but in flight it could be done in 20 minutes. The great advantage of fast flight is the ability to disperse over large areas which in turn means a greater chance of surviving abrupt environmental changes. In the fossil record wings first appear in insects as a double pair and many existing kinds of insects still have four wings (beetles, bees, butterflies). Some insects have lost a pair of wings and now have only two (flies) and a few kinds of insects have lost flying wings altogether (fleas, bedbugs). There is a hitch in flight as a way of escaping predators. Some birds have evolved as predators (hawks) and among their prey are their fellow birds. The same thing occurred among insects. Dragonflies feed upon many kinds of insects.
The only other truly flying animals are bats and they serve us well by eating enormous numbers of night-flying insects, especially mosquitoes. All those other animals described as flying (flying squirrels, flying fish) do not truly fly, they soar and glide. If we sometimes imagine how delightful it might be to fly lke a bird, we should keep in mind to look over our shoulder for the flying predator always lurking nearby.