Tight is not just for keeping warm.
- it can reduce chaffing. Some people will have the left & right thigh rubbing against each other causing chaffing. or a fully wet running short may causing friction with body and cause chaffing. Tight is to reduce/eliminate it.
- compression tight also control the muscle movement and therefore reduce waste of energy to improve running economy. Plus the compression will be better in driving the blood to go back to your heart and will reduce tiring for a very long run (e.g. full marathon or longer).