My read on it is that the traditional approach in CS was:
1. Non-peer-reviewed "workshops" or "symposia". Place for position papers, work-in-progress, etc. Typically screened only for basic suitability for the event. In some cases authors may only submit an abstract of their topic, not a full paper.
2. Peer-reviewed but non-archival "conferences". Place to disseminate properly written up recent work in relatively short form [6-12 pages double-column], get feedback from other researchers, etc. Typically peer-reviewed by 2-4 reviewers, on the basis of a full paper (not abstracts like in some fields). The top 10-40% of submitted papers are accepted for inclusion (depending on the conference's selectivity), and then accepted authors typically have a month or so to revise their papers on the basis of reviewer comments, before submitting the final version for publication in the proceedings.
3. Peer-reviewed archival "journals". Place to wrap up a project or line of work with a publication that takes 3 to 5 conference publications, and integrates them into a full writeup of the project, tied into a nice bow for posterity suitable for archiving in The Literature. Peer-reviewed in the same way as other fields. Typically longer, 20+ pages.
But what seems to have happened is nobody really reads #3, since all the new work comes out in #2, the peer-review standards of #2 are already high enough for practical filtering, and #3 is years behind. And then people (especially Americans, especially in certain fields) just stopped writing #3 as often altogether. The view seems to be that tying several conference papers into a bow might be nice from the perspective of having a sorted-out archival literature, but at the present time, your peers have already moved on, and there are diminishing returns from tying those 3 already-published SIGGRAPH papers into a summary journal article, compared to working on new research for the next SIGGRAPH paper instead. It's even seen a little as "milking" your previous work: if you take your conference publications and turn them into a journal article, some people will assume you're doing it because you need the journal line on your CV for some kind of tenure/hiring committee, since otherwise why would you be spending all this time "upgrading" old conference papers into a journal, rather than writing new conference papers on new research?