Need to change the category or a bunch of posts? Or tags? Disallow comments? Change the author? The page template, page parent? You need to meet bulk actions.
You can bulk edit posts, pages, comments, lots of custom post types (depending on how they’re put together). You can bulk delete other areas (categories, etc.). In other words, you can stop opening each post one by one to change a category or an author. Getting to know bulk actions can save you loads of time.