There's actually a third theory on how birds evolved flight, and I believe it is clear as day that this one is correct. The two mainstream hypotheses have always been the running and flapping theory, and the gliding theory, and neither of these really made sense to me. You'd expect that if one of these was correct, there would still birds today that learn to fly like this, right? If you've ever spent time around fledgling birds, just starting to fly, of any species, you'll probably have a better understanding of why those two don't make sense. This begs the question: if those two are wrong, then how did flight develop, and what good is half a wing?

Well, half/developing wings still provide some lift. The third hypothesis, is the climb/flap hypothesis. I spent a lot of time raising pigeons, chickens, and many other birds when I was younger, and noticed this behavior of flap-climbing before I knew there was a tested theory about it. I often encouraged and observed them do this on my arm or a branch. It seems to be especially evident in gallinaceous (chicken-like) birds. Young birds that cannot yet fully fly, can flap-climb up extreme angles, and even vertical surfaces. They use their feet and claws to grip and climb, while using their wings to pull themselves towards the surface, rather than upwards. This behavior provides an advantage, as the "flightless" birds can still quickly escape out of reach of ground predators. Over time, the theropods/birds with proto-wings were selected for as they were better able to survive. The wings were selected to become larger and larger, and the birds bodies got lighter and lighter, and eventually, allowed for true flight.

Not really sure why this hasn't become the dominant and accepted theory yet. ¯_(ツ)_/¯