If you mean the 138th Episode Spectacular, there were numerous reasons why that clip show was so great, with each act and the episode itself reflecting a different reason.
The episode altogether: The episode can be considered a 30 minute "highlight reel" of a lecture tour that one of the Simpsons writers had been going on around the time the episode came out (I've read about the highlights about one of these events on the Simpsons newsgroup). Hence, for those who couldn't attend one of these lectures, this would serve as a good alternative.
Act 1: With the focus on the Tracy Ullman shorts, this act showed those peole who've only begun to tune into the Simpsons back when it became a thirty-minute program the early days of the characters and how the characters have changed (and the animation have gotten better) as between the first short, the shorts of the third season of the Tracy Ullman Show, and the half-hour show.
Act 2: This act reflects the two big things that people seem to notice when it comes to the Simpsons:
*How Homer seems to get dumber and dumber each season (with the episode showing a montage of Homer, with each season being represented by a different comical Homer clip).
*Smithers homosexual interest in Mr. Burns.
Act 3: This act featured the thing that people have tuned in to see (besides the montage of "hardcore nudity" clips during the credits

): Scenes that were cut from the episode due to time (as well as alternate versions of some scenes). In a time where DVD hasn't been invented, this episode was gave people a peek at some of the great material that had to be cut either due to time or because the writers thought that viewers wouldn't find the scene funny (i.e. the Bollywood parody in Homer and Apu; the robotic Richard Simmons, which I was interested in seeing after reading about it on alt.tv.simpsons).