During my twenties, I competed in triathlons and kept a log of my workouts to track my training. The training schedule was too complicated to put to memory and still know what I was supposed to be doing on any given day. After I stopped doing triathlons, the habit of keeping track stuck and I have kept an exercise log ever since.
Mine is pretty simple, tracking the exercises I do each day and the duration. I also track injuries and make notes on changes in diet or workout regimes. Nowadays I keep the log on a spreadsheet on my iPhone so i can easily update it and look it over.
I regularly review my workouts and, over time, I can see exercise levels and trends, and even lazy periods. If I notice I am fatigued, I can see where I might be over training and if I miss workouts I know what I have to do to get back on track. The log also creates momentum and commitment because I don’t like to see empty days in my calendar.
I consider the log an integral part of a solid fitness regime.