Wow... I could go on for days.
As mentioned, keeping track of exposure protection and weight. Especially important for someone like me who dives everything from a swim suit to a dry suit with several different wetsuits in between.
Recording your memories of dive trips so you can go back years later and look at them. I regret the 10 or so years I stopped logging. Those memories are mostly lost.
Keeping track of favorite dive sites, depths, locations, conditions. You may want to go there again or recommend it to someone else.
Contact information for dive buddies- I try to get an email address for differ