Why to keep a Workout Diary/Log?

Recently I took some advice and started a workout journal. To get started I went out and bought a pocket notebook for under a $1 and a package of mechanical pencils(I don't feel like dealing with ink at the gym) at the drugstore next to my gym. I start out each entry by dating it and listing everything I ate for the day(gives some perspective on what helps/hurts a workout) and how I feel going into the workout(tired, hungry, strong...). The rest I log what exercise I am doing. This differs for each exercise(how many reps/how long) then at what difficulty(how much weight or resistance), and finally what settings(sizing) on each machine. I find this journal to be very motivating to see how I am improving as well as what works and what doesn't.