Why are every Simpsons intro different?
Is there a story behind it?
It keeps you interested before the show!When I was 7 I used to call the bits at the beginning the funny bit and that kept me entertained for the whole show!Its sort of like a little taster before it begins.
Like the words describe it its an Introduction to the episode, kinda trying to get you in the mood before showing the episode
for the mentally challenged the reason is to entertain you from the very beginning of the show ,so that you will continue to watch it ..................etc.
It serves to keep the intro interesting, even if you see hundreds of them it's always new somehow. It's similar in Futurama. And there is no story behind it, they are just random crazy scenes.