I like to log my caches online, mostly because I like to go back and read the logs of other people that find it after me.
I like to see if there are other cachers who often find the same caches as I do, and to see if anyone else that I passed on the trail was also a cacher.
The numbers are also important to me, but only in a competing with myself sense, and to keep track of milestones. Let's face it, there are cachers out there who could kick my butt number wise any day! And that's fine by me.
Also, untill reading this forum, I never knew it was a big deal to some people when all you put in your online log is "tftc" or the like.
I admit I'm guilty of this. Some caches are just unremarkable. If i put my caching experience in every log, it would be just as boring to read, and take me longer. "Walked down the trail that I usually walk my dog on, found the cache" Don't get me wrong, I enjoy every single cache that I find, otherwise I wouldn't do it.
If I had an interesting experience during the cache, or if it was a place that I hadn't been before, I mention that in my online log, but not the physical log. I always thought I was being courtious by only signing my name and date, to save room and make the log book last longer.
This thread has been eye opening for me, I had no idea some people cared so much about these things. Geocaching to me has always been really casual, so long as nobody is stealing the caches, or their contents, I'd be happy.