(The question, stripped down to its bare, insult-free essentials, is "Why do you keep an online journal?".)
I post in my journal, I keep it, because I can. It's many things to many people, and some of those people are even me.
First off, it lets me keep in touch with some old friends that, in all honesty, I probably wouldn't keep in regular contact with otherwise. This is a good thing, and it works both ways - I get to see what's going on in their life (for the ones that have accounts and post) and they get to see what's going on in mine.
Secondly, it serves as a personal record of my life. A lot of entries I do post for the public, but there are a lot I post for myself - many of which are friends-only or even private. It gives me a way to put my thoughts in order, a way to keep track of what's going on in my life, and a way to retain some sense of what I'm going through. Sometimes it can be almost like a form of therapy, where I can work through things that are bothering me. Putting your thoughts on paper forces you to think about them, order them, etc. This is no different.
Thirdly, and the reason I chose LJ in the first place, was because I wanted to keep my journal online but have protection for it. Specifically, I wanted to start a journal but didn't want to keep a paper journal, as I knew I'd never keep it up. I wanted it to be online so I could access it from anywhere, but to have some security so I wouldn't have to worry about random people reading private entries. I wanted it to be easy to update as well. My original plan was to write my own set of cgi's and password-protect the whole she-bang, somewhere on my webserver, but I figured I'd look first and see if someone else had done the work for me. I found a few options that I could install on my box, but LJ seemed to have such a nifty featureset that I figured I'd give it a shot.
Fourthly (I hate that), it gives me a chance to get input on things. A lot of times people will comment in my LJ and will say something that makes me think, that brings me a new perspective, that brightens my mood, that challenges my thinking... these are all positive things.
Fifthly (oh Lord), it provides an interesting way of meeting new people, and an interesting glance into their lives. One of the great parts about LJ is how not only is your journal up there for reading but so are hundreds of thousands of others, including personal journals, work journals, dream journals, communities, all sorts of things. To really get to access the whole thing, though, you need to have an account, and having one without using it seems almost silly.
Sixthly (heh), since people regularly read it and comment on it, apparently someone other than me gets something out of it as well. Whether it's amusing or interesting or thought provoking or just a way to pass the time, someone seems to like it, and that's a nice thing.
Finally... I think... well, all of that buttock-clenching really provides some nice tone, and chicks dig a nice butt. So that part can't be overlooked.
Keeping an LJ is fun, it's therapeutic, it's useful... it's been a great thing for me, in many ways. So that's why I keep it, and that's why I'll keep on keeping it.
Incidentally, to the anonymous person: get an account, try it out, and next time have the balls to post as yourself. Cowardice is easy, I know, but I rarely bite anyone's head off for an honest question (although next time, leave off the insulting bits), and even if you're someone I loathe, I can still be civil... most of the time.