How are Functors useful?

Question

We know that any generic type F[_] withmap method, which complies to some laws, is a functor. For instance, List[_], Option

Answer 1

I don't know Scala, but in Haskell, the Functor class is essential to defining Van Laarhoven-style lenses:

type Lens' s a = forall f . Functor f => (a -> f a) -> s -> f s

These lenses are typically defined for specifically-related types s and a, but it's essential to their utility that they work with an arbitrary functor.

Functor is also important in its role as a superclass of Applicative and of Traversable. When working with these more powerful abstractions, it's often very useful to reach for the fmap method.