URL removal explained at Google Webmaster Tools

Why would anybody EVER need to remove pages from their site from the Google Index? And why would this be beneficial?!? Isn’t everybody fighting like crazy to be IN the Google Index?!! Man, I can think of a LOT of reasons… ┬ádiscontinuation of a product line, marketing change, branding change, internal company change, new division acquisition… just lots… and just like a bush, if you PROPERLY prune your site, it will grow MORE!

Official Google Webmaster Central Blog: URL removal explained, Part I: URLs & directories.