They control the reaction within the reactor, and in newer reactors can be used to regulate load. France & Germany do have a load following system in place for their nuclear reactors. It's not a simple matter of turning off a reactor, as this is a large maintenance task. Various mechanisms and regulations have been developed to handle load following for a Nuclear plant. There are still negatives to doing this, such as reduced efficiency, additional operator training, and additional wear on control, fuel and the plant. Positives likely outweigh the negatives, but the reactor must be designed to do load following and regulations need to be in place to ensure it is done safely. It is another thing to consider when talking about the cost of nuclear.