Why are conference papers so important in computer science (CS)? From my understanding many CS conferences are highly selective and the papers are peer-reviewed. The CS field also seems to publish traditional journal articles. I do not understand the difference between the two and in particular I am curious about the advantages of having two different publication streams. The type of things I am curious about are:


What is the difference between a paper published in a top CS conference and a top specialist journal?
Is one more prestigious than another and if so how do they line up?
Why not just publish conference papers as a special issues of a journal?
I'm afraid this answer may be unsatisfying, but at its root, it just boils down to the culture of the field, as determined by early choices in its particular history.  Now that it is well established, practices have developed that cement it in place, including clear distinctions between what goes in a journal and what goes in a conferences.  

Michael Ernst has a very nice write-up of the typical way a computer scientist thinks about conferences vs. journals.  In short:


Conferences are fast, higher status, higher selectivity, and higher visibility.
Journals are where you put a review article or a final "extended version" of a paper (the typical threshold is "at least 30% new material").  


Journals are also important for getting tenure at lesser institutions, which apply the same standards to biologists and computer scientists.  This also further lowers the perceived status of journals, since the best departments are less motivated to publish there than the worst.