Hi Harryvan
As a physician on the "receiving end" of many of the questions over the years I think there are a variety of reasons.
Some may want a second opinion, some can't afford to seek an opinion from their doctor in a fee paying set up (or want to firm up their ideas before going along to the doctors office) and some have reached a situation where they no longer respect or trust the opinion of their doctor.
There are probably more reasons than that and I'll be intrigued to find out what others have to say about why they choose to seek help this way. Part of it may well be having a sense of connecting with a wider community who they feel can share their experiences.
It's certainly an interesting question.
I almost always include a statement in my answer that the question setter should seek help from their doctor and that conditions cant be reliably diagnosed online. I hope most take heed of that.
PS - if you think there is a lot of this kind of question on Yedda then take a look at Yahoo Answers where I've seen people post genuine questions saying things like "I've got a crushing pain in the middle of my chest - what should I do"?