Like many Emacs users, I do almost all my file manipulation (rm, mv, cp, etc.) in dired. It’s just a lot easier to have a listing of the directory in front of you and mark the files you want to act on. That’s not even mentioning writable dired, a thing of beauty in itself.
Over at (or emacs, abo abo gives us a bit of Elisp that let’s us mark some files and run the Unix du on them. He maps his function to z in the dired-mode-map so all we need to do is mark the files and type 【z】 to get the sizes. It’s an easy enhancement but it gives us one less reason to leave Emacs (or even invoke a shell) to get information about our files. Very nice.