We are predisposed to think of feathers as equipment for flying. But seeing all those flying "dinosaurs" flitting about our yard is misleading.
Reptilian scales are basically fish scales that have been greatly toughened to control moisture loss, allowing colonization of the land. But if you are a non-big dinosaur, thermal regulation is a significant problem. Feathers are basically scales that can be fluffed up or laid flat, to varying degrees, giving different insulating profiles, at the cost of possibly losing some moisture, which many dinosaurs could well afford.
The feather more appropriate for flying could have been variants that were big for display and could lay very flat. But the original purpose was not flying. Flatness is possibly desirable for: reducing insulation when desired, streamlining the body if traveling quickly through brush, making big visual displays with relatively light equipment. However a small dinosaur that jumped around trees would find that large flattish feathers would give it added control over gliding descents, which is a fabulous thing if you are in a hurry.