I'll echo a few here.
Log the first 100 dives.
Collect the stamps.
Write about the dive (what you saw above/below).
Dive profile.
Water temp.
Equipment (rental/personal) - you never know what you may like and look back at 'I want to buy THAT one!' When the time comes.
Weights & exposure suit.
Anything AMAZING and OUT OF THE ORDINARY!
Once you have that, it's up to you if you'd like to continue.
By this point, you'll have enough logged dives for most any course, lots of stories and memories.
If you carry on logging beyond that is up to you. Most places don't require actually seeing a logbook, but for some reason really want to sign one off, especially after a course before giving you your earned card.
Me personally, I log every time I blow bubbles. Electronically and downloaded from the bottom timer.
I log who/where/what/when and the how (gear - exposure, what set of tanks, weight - lead, batteries, etc).
I enter in pressures, evaluate the profiles for later trips. One common one is, is it worth the extra $$$ for this remote location for fancy gas? Or am I going to be OK with a Nitrox/Air dive?
Same with sizes of tanks, am I OK with an 11L, or do I need something bigger?
I also write about the dive sites, where things are, what works, what doesn't. So next time I go there or someone asks, I can have an idea about where to go or what to do while there (where the octo/wolf eel den, what side of the island is better for sea lions, where is that small wreck, island cave, feature etc).
Also charter costs, gas, transportation, accommodation, locals, foods, etc.
While this may sound like I've written a novel about every dive or trip. It really isn't. The initial logging of a location or site may be more in depth than the rest, the information is there. Subsequent dives/trips just build onto what is already there. I leave that to the bloggy things and other social mediums
_R