Because things can happen that cause you to want to change it. For instance if one of your players were to suffer an injury in a game, so you wanted to train something else for a few of you other players instead of wasting the week by training a hurt player who didn't get enough minutes. Also, maybe just the simple fact that you changed your mind on the direction you want to train your player/players due to forum help from other managers. The list goes on and on but believe me it is a good thing they give you the option on changing it.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs