It’s a fact. You just won’t get much done in your practice session unless you know what you are going to practice before you sit down at the piano.
Life can be busy and practice time can be precious. I know that if I sit down at the piano without a plan I’ll start noodling around and next thing I know half of my practice session is gone! For that reason I’ve always been a list-maker. There’s almost always a list of some sort on or near my piano.
When I was in school and at my most productive I was super organized about it. I was on a mission to learn to play. I was specific because my time was limited. I kept a practice schedule on a piece of paper that listed all the projects I was working on in rows down the left side of the paper, and the days of the week in columns across the top. So if I did something on Monday I checked it off under the Monday column, when I sat down on Tuesday I knew immediately to start with something else. No time wasted. I updated the schedule every week with what I wanted to accomplish and looked honestly at what I got done the week before.