This is an essay, essentially saying that I write in Wikipedia both for myself and for others.
For my part, it's not about prestige, because Wikipedia does not offer it. Rather, it's about offering information to the people. I rather have millions of people reading my work than letting it rot in some university archive with my mere name on it. After all, this is what people read. If a person wants to read about AIDS, that person likely googles for AIDS, and it's the Wikipedia article that comes up first. Subsequently, it becomes an obligation to give that person the relevant and the correct information. That's what we do here.
For my part, I also get is an expansion of my own memory - Wikipedia has been my notebook throughout my entire university studies. Once it is there I can find it again with just a click from any computer or my mobile phone. Especially, I'm able to find it fast, even in the hurry that is common in the hospital wards. Also, its policies on reliability and verifiability force me to learn evidence-based medicine. In many cases, it has just been a coincidence that the rest of the world can make use of the information as well.
Sometimes I actually get the feeling that Wikipedia is like my own little robot apprentice, which I try to teach whatever it doesn't know. But actually, just taking the example that the Wikipedia database was a major component of the "memory" that the Watson system used to win Jeopardy!, what is written in Wikipedia will definitely be part of future artificial intelligences.