The exchange of articles, actually pages, is held for the purpose of improvement of a visibility of a site in a search machine for increase of the quotation index or the PageRank index.
Essence of an exchange is that you give your specially prepared advertorial to an exchange system or to a partner. The advertorials are placed in separate pages on a partner site.
Such a method of promotion of a site will work well if the article matches by subjects with a partner site.