just as Sodom and Gomorrah and the surrounding cities, which likewise indulged in sexual immorality and pursued unnatural desire, serve as an example by undergoing a punishment of eternal fire.
Here is another example that God does, in fact, judge.
It is easy to think that God doesn't do this. Many people like to think of God as this great big old guy, with a long beard, sitting up in the clouds. He is always one step ahead of you, but as far as judging people, he's not really going to do that. Yet the Bible is filled with examples of God doing that exact thing.
In the case of Sodom and Gomorrah, He is bringing down actual fiery judgment.
Jude says that this example is trying to lead you to understand that there will also be an eternal fiery judgment one day.