Benefits to logging dives, if you take time to log all meaningful information, include:
1. Proof of diving experience for purposes of continuing training prerequisites, or proof of experience to show a dive operator so as to be allowed to go on certain dives;
2. Creating of a data base of weighting for various equipment and conditions;
3. Creating date on air consumption rates based on depth, conditions, and purpose of various dives;
4. Creating a data base of dive sites and their topography, fish life, currents, and other information for repeat trips or sharing with other divers headed to those sites;
5. Noting information on other divers participating in the dive, and contacts for them for future reference;
6. Any other relevant information.
My dive log has resulted in a weighting chart I keep update, so I know what weight is proper when wearing what equipment in the conditions I will be diving. This is a hugely useful bit of information. I alos use the log to compare the condition of a site to the last time we dove it. For example, on a recent trip to Akumal we dove several sites we had visited a year and two years before, and I compared notes in particular on lion fish spotted and at what depth, and was able to note the decline in spottings and the deeper depths where those we did spot were found. We were able to share this information with a local group that is involved with eradication of the lion fish. On laswt use of a dive log that is maintained with substantive information and date. It helps me keep track of the performance of equipment, and the time under water that equipment has been used and how it is functioning. That helps in the determination of when it is time to have a piece of gear serviced, hoses replaced, and so on. I am a BIG fan of logging every dive. I have a blog post on that you may want to read.
DivemasterDennis