Lack of a plan means that you have no (known, valid) reason to say no. Requests, demands, suggestions, obligations, interruptions come at you. You stutter and stammer and end up obligating yourself to 157 things you don’t want to do.
Without a plan, you have no direction. You have not claimed your time, stated your purpose, so others feel free to claim it themselves. (I’m not saying this is right, but it’s going to happen.)
Not having a plan means making more decisions. Decisions take energy. When you have to make a bunch of decisions just to get your day started, you’re using up energy on trivia. You could be using that energy for things that are actually important to you. You have a limited amount of energy to use each day. When you have a plan, you can use more of your energy on your priorities.