There are many different reasons for logging dives. For one, it is proof to a dive boat of your experience. Also I find that most people who are occasional vacation divers forget how much weight to wear. If you put that information in your log book also then you have a reference to remember when I dive in the local quarry in July I wear this much weight, when I dive in California in November I wear this much weight, and when I dive in the Virgin Islands in Feb I wear this much weight. It is also a great place to look back and remember. Hopefully some day you will have so many dives under your belt that you will not be able to remember them all. You can look back to your last trip to the Keys and remember good spots, not so good spots, etc... I also use my book as a signature collection of all the different people I have been able to dive with. I fill out my logbook as I teach my students how to fill out theirs. This shows them the importance of doing their logbooks, and I have them sign my book for me too. Some people get out of filling out a logbook after they have aquired so many dives, but I still keep mine up and will continue to do so until my diving career ends.
Shelly