"The dullest pencil ... remembers better than the sharpest mind." My co-worker, and friend, Ted Cramer, says that all the time, and it is very true. We all like to think we can remember whatever it is we are supposed to remember without having to write it down. If we are honest with ourselves, we know better than that.
I have made enough trips to the grocery store at this point in my life to know that if I go, I better write down everything I need to pick up. Having a written list saves the embarrassment of needing to call my wife before I get in the checkout line to make sure I got everything, because I know I missed something (I hate that). Having a list is also the only absolutely, positively, 100% way to be sure I won't have to make a second trip (I really hate that). And so, I write things down because my "forgetter" works a whole lot better than my "rememberer" does.