I have kept one at my cabin in Colorado for over 20 years and one at my hunting camp in Nebraska since I bought it 4 years ago.  Keep track of game sightings, weather and anything else of interest. Fun to look back.

I started one back in '94 when I began gun hunting deer. I filled in the off-season times with thoughts about hunting, gun purchases, and eventually chronicled my moving into archery. It's pretty neat when you can read back and find the exact day you bought a bow, where you got it, what you paid, or what the weather was like during a hunt. You DO forget. But when you read back, the memories come flooding forth.
Last year's hunt I chronicled on another forum; if you're not a writer it's really easy to sit at the keyboard (as now) and type in the details. Readback is Total Recall!