(And hey, I can also use it to document any new ideas I have--if I ever need to go back and say, "I came up with it on this date," I'll be able to.)
Here's the thing: for me, the real purpose of keeping a date book--a diary--is to help me keep the days from sliding by unused. I have this terrible habit of letting the days drift by in a haze of unproductivity; my current fear is that I'm going to open my eyes and discover that it's June. It's like mountain climbers with their stakes--I feel like I'm staking each day in place to make sure it doesn't slide away, and maybe it'll push me to do more so I'll have something to write down.
... Except for today, in which I am too tired to do linkspam. Maybe I'll do it late tonight, I don't know. It's just been one of those days.
ETA: Some quick pictures of my planner: