Hi Frank,
I'll give this a shot, though I'm sure others would explain it better than I.

A pilot must keep a written record of whatever flight time is required to meet the requirements for a new certificate, or rating, and also to show he has met recency requirements (currency -- like the number of landings to meet night flight requirements, etc.). You would also record your instrument time, and approaches -- also required to act as Pilot in Command under Instrument Flight Rules. There are other "required" times, also.
Of course, MOST every pilot we know writes down all flight time, because why cut yourself short and only write down what s reqired for the FAA needs? Usually, insurance requirements will be higher than the FAA minimums for certain types of flight... for example, the FAA just requires you to have a tailwheel endorsement, but your insurance company may want to see 25 hours in a certain type of tailwheel airplane before they'll insure you.
Pilots have differing views on whether to write in personal items; I was told by a check ride examiner once, who eyed my boring, spare lines, "You might want to add in some notes there -- some day you'll want to look back and KNOW what you did on this flight." So, I started doing that and was glad... even a little line like "amazing lenticulars all along the Divide" will be enough to immediately remind me of just which flight that was.
I know people with so much flight time ("Chuck, is that you???") that they haven't written any of it down for ages... they keep the bare bones records required to show they're legal, and that's it. Of course, they does NOT apply to some of us peon chickens in the roost!! HA.

I use a home-made sheet for each flight, and then stack them in incredibly messy-looking piles when the season is over. I should join the modern world and find a good electronic logbook, and get it all in one place, but... I'm kinda slow.

I hope this sheds a little bit of light on logbooks. I sure hope no one else's looks as disorganized as mine!!