I've been doing this since day one in feeders. Anything in my daily routine that is out of the normal, I make a note. When I first started doing it, I made COPIOUS notes. Way more than I needed to. Over time, I learned to streamline the notes and only note the things I knew they would ask about. You quickly learn what they will ask about.
Starting out, the main purpose is to CYA. If they have a question, you have an answer. A real answer. But I learned what the real advantage of keeping a record of my day: my book had way more power than I did.
In feeders, one of the on-road supervisors nasty little tricks, is to bring you into the office and ask you what caused this or that delay a week ago. Now, we're not inventing the wheel in this job, and with rare exceptions, today, tomorrow and a day three weeks ago all look the same. So, unless you keep notes, or have a photographic memory, your answer to their questions looks a lot like your finger scratching your head.
Here's where the power of the book really shines. If you're meticulous about doing things by the book and the methods, like I am, things take a little longer. So when they would say we need to see you in the office, I would say, "Let me grab my book." After four or five times of this routine, my supervisors would ask me into the office and would look at my book instead of me. It was almost comical.
After a while, they pretty much left me alone. My notebooks were also a good reference for myself. They are a good snapshot of how I've done this job. And as a long-time cover driver, I always have a good feel of where I'm going from where I've been.