There are a couple that come to my mind when i used them working for an agency. I remember one client had some temporary pages that didn't want to get indexed, explaining certain problem with a product at that time. We wanted the page to be live, but didn't want the problems that the product was having to show up in the search engines since it was just temporary.
Also, pages that are targeting same keywords that you dont want to erase or redirect and instead want to keep them live but at the same time you dont want to compete with the other main page. You just block it to the search engines.
Hope this helps