They restart to fix bugs as well as when they push out an update. Most of the time the servers are up, and the restarts need to happen to keep the servers functioning. If the restarts happen, its so people can continue winning in the future.
Also, not every force disconnect is from the server restarting. It's only a restart if it says its a restart. Sometimes your internet slows and boots you off. Other times minecraft crashes. Just know whats wrong.