They are specifically there so that you can write zero-cost abstractions. Yes, people who are hardcore C programmers may not have experience with more advanced type system features, but it's all in service to the goal of writing expressive code that goes as fast and as safely as possible.

As a practical example of higher kinded types, you can't say "this function takes an Rc<T> or an Arc<T>. I just want something refcounted, but I don't care about how." HKT would get you there.

Another common problem it would solve is "I don't want to write both a &T and &mut T version of my function."