I keep a log for some of the same reasons as others have stated. It is also nice to have a reference of water temps and other conditions at various times of the year. My log over the past 10 years also includes weight worn in different conditions - nice to know information when returning (assuming similar gear). Not that it has changed much over the years, but I also have air consumption data for most of the dives. Again, understanding variance in air used in different conditions is helpful in planning. I'm a stats geek... When I can keep up with it, I like the comments section to record what was seen and relative merits of the site. Again, this can be helpful in guiding which sites you want to hit on a future trip. Logging isn't for everyone and I have to admit that I go through dry periods where I log only the raw data.