I haven't been doing this too long, but I think taking notes drastically increases the effectiveness of my workout; when I can look back at the previous week(/s) what weight/reps I did and can increase the difficulty slightly or not depending on how I feel that day, so that I can do close to the max of my abilities immediately instead of having to waste time and energy on trial&erroring.
eta: and it's also an extra incentive to me to do just that little bit more to be able to record progress to the last time I did a specific exercise.