In order to keep track of your blade use?
I just started one and find that it helps a lot. Especially if you have a lot of razors in rotation.
Before, I had only one blade at a time in circulation and rotated it amongst my razors. I still had to keep track of how many times it had been used, though. With a log, I can have multiple blades going, and different types, depending on the razor. Plus I don't have to track it in my head, freeing up valuable mental space.
I thought keeping a log was way over the top when I started with DEs, but it helps a lot.