Why do we need journals anymore?

There was a recent email on PLANET Google Groups asking about what the ‘top journals’ were in the field. I wrote a response, while not directly answering the question. This post is an elaboration/edit of that post. Do we need journals any more? I say this as/has been an associate editor for journals and appreciating the full scope of publication infrastructure that publishers provide. I also appreciate the need for cognitive shortcuts to quickly evaluate the value of research by looking to the reputation of the journal.