Sometimes these closures have more stuff in them than you could conveniently put in a url. For example, they might refer to another closure describing what to

Sometimes these closures have more stuff in them than you could conveniently put in a url. For example, they might refer to another closure describing what to do next. When you try to do something that requires you to be logged in, for example, you're given the login page, with a fnid referring to a closure describing what you were in the middle of doing. That's why after logging in you're able to continue doing whatever you were doing.