Examples help in making an abstract concept easy to grasp. If someone tells you, "Hybrid cars last longer than petrol cars", you might take a bit to think about that, but if they add "For example, the Toyota Prius comes with an eight-year warranty", you're probably more likely to agree or disagree straight away.
When you make a statement, a reader has to accept, reject or ignore it. This means understanding it, which often means coming up with their own examples to relate your abstraction to their knowledge. Save them the work (and make your writing more persuasive) by including examples.
Examples also help clarify confusing or potentially ambiguous concepts. For instance, when you say, "Professional courses, such as medicine and law, often require a degree as well as a licence", by stating examples (medicine and law), you're making your definition of professional courses clearer, so you and the reader are on the same page.
Be careful, though: examples don't replace explanations. You need both—the definition states the abstract rule, and the example gives one instance. If you only give the example, you risk your audience forever thinking in that box.