I used to play my calendar like a game of Tetris.
I would stack obligation after obligation for every minute of my waking life. Go to the gym, then work on this, then meet with that person, then meet my friend and so on.
Letting my calendar be my master put me on auto-pilot. Not needing to make decisions about what to work on can be great for some things in some situations. However, deciding what to do with your time is one of the most important decisions someone can make.
Therefore, I prefer to make conscious choices about the best way to spend my time at any given moment.