I enjoy posting as I like people. I am sociable in 'real life' and enjoy being with others, although I also need time alone.
I sleep badly, and like that online communities are there all day and night, and are asynchronous, so I can reply to something posted hours before, and it doesn't matter if I post when there is nobody else awake in the middle of the night, as people can read and reply to my ramblings in the morning.
I also like the anonymity. I can say what I think, and hear what others think without the social niceties that prevent 'plain speaking' offline. That doesn't mean I hide behind a keyboard, just that there are things that don't come up in 'polite conversation' (eg money, politics, relationships) and I like learning what people actually think, as opposed to what they say in person. I suppose it's similar to why I like listening to Any Answers and other 'vox pop' type programmes.
I also like the fact that we get to know one another a bit. I can often work out who is speaking without looking at their name, and even though I don't know the poster in person it gives a sense of community. I like posters who can disagree without being personal, and who don't carry grudges from one thread to another.
I haven't met anyone, but I have exchanged emails with posters past and present. I think it's possible to have friends you haven't met, although I realise a lot of people think that's odd.