It can be a coach and a crystal ball. It can provide a wake-up call and a slap on the wrist.
A training diary is one of the best tools you can use if your goals are to improve. Performance trends and patterns that cannot be seen by observing one day at a time become clear in a diary.
Your resting heart rate for one day does not tell you much, but observing the pattern of rise and fall against the background of a training schedule can red-flag fatiguing workouts and identify the number of days required for proper recovery.