It is easy to remember that a pair of skis felt good on a certain day. It is harder to remember whether the side edge was 88 or 87 degrees, which stone you used, what the base looked like before waxing, and whether the snow conditions were hard man-made snow or soft natural snow.
The service log keeps that experience from disappearing between ski trips. Do not write a technical report. Note down enough to compare: what the ski needed, what you did, how it performed, and what you would change next time.
After a few ski days, you will see how often that particular pair of skis needs to be touched up with a stone, which glide wax covers most conditions, and whether a certain problem keeps returning in the same part of the base or edge.
Give each pair of skis a name or number. Also write down the side edge and base edge after a full service. If you leave your skis with a workshop, you can ask for the angles and enter them in the log.