Why do reviews of articles often start with a summary of the article? In my field, peer reviewers often start their review with:


  This article reports on a study that did X, using Y, in the area of Z. It found ...


This seems like a waste of time and effort to me (and I never do it). Why is it done? To prove the reviewer has read the paper?
Reviews are communications to the editor, and an effective review tells the editor what the article is about (in that reviewer's opinion), using significantly fewer words than authors typically use. If an editor has to process a couple hundred submissions per year, it's not possible for him/her to carefully read every paper, so the editor will especially care if the reviewers agree on what the paper purports to show and whether the paper actually shows it.