CanBuildFrom is essentially a typeclass in Scala that is used for converting between collection types. Ignoring the coherency issues associated with using implicits for typeclasses, it makes the implementation of the collections library significantly more complicated and causes issues with type inference.

In the rare cases where you actually need to convert between collection types, it's usually trivial and more efficient to do it manually. Essentially they've added tons of complexity for negligible benefit. One of my main gripes with the Scala collections library is that it's overly complex and that this complexity has been the source of hundreds of bugs.

If there was any real benefit to the way it's currently done they wouldn't be hiding the true type signature for things like `List.map` in the official Scaladocs.