Why was the ShoemakerminusLevy 9 impact so important to astronomers?

It convinced astronomers and politicians to treat the real chance of the earth getting destroyed by impact as a possibility.

The idea of a large comet or asteroid slamming into a planet was science fiction, until we saw it for real. A Mt. Everest-size chunk of comet created a fireball about as big as the Earth on Jupiter.... It's not science fiction any more.

*That's* why it's so important.

It was important for two reasons: (1) it gave some insight into the dynamics and composition of comets, because we could watch it breaking up, and (2) by plunging into Jupiter's atmosphere, it stirred up material from the deep layers of the atmosphere that we otherwise never get a chance to observe. As a result, it told us a lot about the structure and composition of Jupiter.

Answers to this question about how comets might crash into the Earth are wrong. That is not why Shoemaker-Lavy was importnant.

Because nothing like that had ever been seen before. A large comet smacking into another planet. We had a chance to see the effects of a comet impact on another planet. This helps give us an idea of what might happen if one were to strike the earth. It would be bad.