For example, if your blogengine has an archive without stable urls, you don't want that searchengines find an archive-page with many articles instead of the art

For example, if your blogengine has an archive without stable urls, you don't want that searchengines find an archive-page with many articles instead of the article itself, because it is possible the wanted article is already on another page. The typicial seo-way, as far as i know, would be to noindex the archive-pages, but you of course could also make the archive stable. Both ways are an optimization for search engines.