The granularity with which you can recognize trends and speak to exactly what your customers are looking for in copy is far enhanced by keeping a document like this than memory or mnemonics.
I guess I mis-read the post because it sounded like the most important document of my life, not for product management. I will agree that it's important when you're a business and you want to keep tabs on your customers and their use of the product but I still doubt the point that it's the most important document (except for product management or customer development or customer research).
This is an interesting point. I tend to document Q/As that take me more than a couple of seconds to answer. If I logged every time I looked up a question in my notes file, would I get a sense of cycles or trends?