Every curling team has a coach. The main purpose of the coach is to help the team develop a strategy for the games they will play. At the national and global level, players do not need to be coached on how to throw a rock. The coach has a very limited role during the games.
During the game, the coach is only allowed to go down to the ice if the team calls a timeout. At this time, the coach will advise the team about the types of shots to make, and what to expect from the other team's shots. Otherwise, the coach remains away from the ice, and can have no communication with the team.
While the coach is watching the game, they may keep track of the types of shots being thrown by the team, and how effective each shot was. Coaches also keep track of how individual rocks are behaving. Every curling stone has its own slightly different set of characteristics. The coach will then communicate this information to the team at the halfway point in the game. This gives the team the rest of the game to adjust based on the feedback they have received from their coach.