People often talk about the internet as the great democratizing of information. But not only is a great place to go and grab information it is a great place to go and teach. I spend lots of time messing around with tutorials. I love to learn new stuff and I really love to learn it from people who are sort of like me–people with information and no one to share it with, so they put it up on the internet.
Sure there is a nice small measure of fame that you get from posting online (and it is a very small measure) but mostly I think people share because we are wired to share. We are wired to teach what we have learned. Evolutionist would say that this is a survival instinct. I say it is because we were created in the image of God and God loves to share what he knows with his people. He loves to teach and to help us to learn. I think everyone likes to share what they have learned and if it is something you are passionate about like to help other people to understand and be passionate about it too.
Now granted some people are better at presentation than others, but I still think that all of us want to share our knowledge with others at least to a small degree. That is why I love the internet. It lets people share their knowledge on a global scale.