Dinosaurs sit at an evolutionary point between birds, which are warm-blooded, and reptiles, which are cold-blooded, our results suggest that all major groups of dinosaurs had warmer body temperatures than their environment.

Robin Dawson

It's possible that dense feathers were primarily selected for insulation, as body size decreased in theropod dinosaurs on the evolutionary pathway to modern birds, feathers could have then later been co-opted for sexual display or flying.