In the intro it always starts the same, the burger, Bob and his family, the restaurant, then the bad things that happen to make it his grand re re re opening. It usually goes fire, rats, then a car crash into a telephone pole which falls into the restaurant. Which leads up to the pilot episode. But in every episodes intro they call a different exterminator, leading to a change of the future. I like to think of it as a bit of a twist on the butterfly effect. How going back in time and killing a butterfly could have altered the future substantially. Except they're not going back in time, but instead in a different universe and who they call for their extermination changes something. Like each episode shows the day(s) that were altered by their choice in exterminator.

That's what makes it funny.