I enjoy debunking. I like fixing things, and bunk is something that needs fixing. I like presenting a good case, solidly backed my facts. I like figuring out where in the chain of reasoning the bunk purveyors went wrong, or which facts they base their argument on which are not correct, of which parts of history they might be overlooking, or some telling context they may have missed. It's fun, like solving a little mystery, a treasure hunt, a little Sherlock Holmes action.
There are two parts to debunking. First you've got to research the actual facts and then secondly you've got to present those facts in a convincing manner. Both of these can be a bit of work. The research phase can involve a lot of googling, and reading. The presentation phase can require some careful consideration. Both are mentally challenging, like little puzzles in themselves. A good mental workout.
You learn new things
By definition research will cause you to learn new things. But as well as the minutia surrounding the particular conspiracy, you are also going to learn some useful and interesting science, math, and history. Generally it's pretty fascinating stuff. Even if it's not fascinating, it's pretty useful for future debunking.
Sometimes it helps people
People fall for conspiracy theories, pseudoscience, and quackery. They always will. But by presenting reasonable expositions of the facts you can help some people from falling for the scam, or damaging their health, or even save them some money. Harm is being caused here. The web site What's the Harm documents thousands of cases where people have been actually harmed by bunk.
Sometimes something is just wrong.
Things that are wrong make your blood boil, just a little. It's annoying to see statements in the media that you know are just plain wrong, and it would be great to be able to correct them, at least a little. Like a bad tooth you can't just leave it alone. You've just got to do it.
Science is great, it's given us wonderful inventions, making life longer, easier, and with more free time. Reason is also great, as it allows us to figure stuff out, and it's the basic underpinning of science, as well as a great way to help run society. So we've got these good things, science and reason, and yet there's a movement, particularly in the US, that's distinctly anti-science, and anti-reason. This movement comes from many directions, but is mostly associated with political and religious extremes. I think that this is de facto a bad thing, and that any little thing we can do to increase the usage of science and reason is a good thing. Debunking is a good thing, because it removes bunk using science and reason, so simultaneously shows people the truth of a particular situation, and demonstrates the utility of science and reason.
That is, if you do it right.