Why Put Scene's After Credits?

Why is it that studios or directors put a scene at the very end of the movies credits?  These scenes are referred to as post-credit scenes, easter eggs, stingers, tags, and credit cookies, which ever word you prefer really.  I'm just like every other die hard movie maniac, I sit and wait for the scene every time I know there is going to be one.  Marvel is the latest studio to do this, and its not as though I'm complaining about the scene itself, I would just rather have the scene show up after a dramatic pause at the very end of the movie or even after the first initial post-credits role, but why make everyone wait for ten minutes just to see a thirty second to two minute clip? 
Now they first started doing the post-credit tease in the sixties at the very end of a James Bond film with the phrase, "James Bond will return in...," then having the title of the next Bond movie show up, giving people something to look forward too. 
They didn't start putting scenes at the very end of the credits until 1979, which began with The Muppet Movie.  In the eighties, these scenes could be viewed at the end of some of your favorite comedies, one being one of my favorite post-credit scenes, Ferris Bueller's Day Off; not to sure if you could call it one due to the fact that a scene was rolling the entire time the credits were rolling, but having Ferris come out wondering why we're still watching comes to mind everytime I wait for a post-credit scene. 
Here and there, they would pop up in movies like the Pirates of the Carribean films or X-Men, but they didn't really become popular again until recently with Iron Man when we see Nick Fury standing in Tony Starks house wanting to disguss "The Avengers Initiative."  Most post-credit scenes, before Marvel films, were mostly for a gag or a laugh.  When Marvel came along they used these scenes to connect an upcoming Marvel film with the feature you just watched; this was all leading to the Avengers film, which just came out this weekend.
I still like these scenes, I'm just asking for studios to place them before the credits role, or maybe even during the credits when we're walking out of the theater, but not the very end for the love of god.  By the way, there are two scene's during and right after the credits in The Avengers, so stick around for ten minutes after the movie.