As everyone knows, this is the most rational and non-obnoxious way to think about incentives and disincentives.

In all seriousness, coming up with extreme, contrived examples is a very good way to test the limits of moral criteria, methods of reasoning, etc. Oftentimes if a problem shows up most obviously at the extreme fringes, it may also be liable, less obviously, to affect reasoning in more plausible real-world scenarios, so knowing where a system obviously fails is a good starting point.

Of course, we're generally relying on intuition to determine what a "failure" is (many people would hear that utilitarianism favours TORTURE over SPECKS and deem that a failure of utilitarianism rather than a failure of intuition), so this method is also good for probing at what people really believe, rather than what they claim to believe, or believe they believe. That's a good general principle of reverse engineering — if you can figure out where a system does something weird or surprising, or merely what it does in weird or surprising cases, you can often get a better sense of the underlying algorithms. A person unfamiliar with the terminology of moral philosophy might not know whether they are a deontologist or a consequentialist or something else, and if you ask them whether it is right to kill a random person for no reason, then they will probably say no, no matter what they are. If you ask them whether it's right to kill someone who is threatening other people, there's some wiggle room on both sides. But if you tell them to imagine that an evil wizard comes before them and gives them a device with a button, and tells them that pushing the button will cause a hundred people to die horrible deaths, and that if and only if they don't push the button, he will personally kill a hundred thousand people, then their answer to this ridiculous scenario will give you much better information about their moral thinking than any of the previous more realistic examples.