You get actual feedback from developers on your code, which is useful. Yes having your sites/apps do what it is supposed to do is important, but you need feedback to learn industry standards/best practices and other gotchas that are much harder to learn on your own.
The teaching content provided by Free Code Camp tends to act more as an introduction than a solid learning resource. Unless you already have some prior experience, you won't have enough knowledge to get through the coding challenges and development projects without using outside learning resour...
Right from the start, users write code. They start small in an environment that has constant feedback and gradually progress to more complex concepts. Users can see code results instantly, giving great feedback.
Codecademy is great for providing a hands-on approach, but there is a lack of context regarding how to start a project in real life. While users may learn how to code using Codecademy's interface, they may not have any idea how to code independently.
Rather than merely test for code correctness, Exercism uses peer review to improve general programming techniques. Users are encouraged to comment on others' solutions, and refine their own based on feedback.