Today most coaches stand on the sideline the entire game. They feel standing allows them freedom to move up and down the sideline to yell instructions. I am sure they feel their activity as a coach can help them “energize” their players, and make it more likely that as coaches they are heard.
A major problem if the head coach stands the entire time on the sideline, they often shut their assistants out of the game. An assistant may see something but not be able to really share it until a time out, then they have about 15 seconds to express their thought before the head coach needs to go into the huddle.