What is the reason of separation of module implementation and signatures in OCaml?

[bluddy]

@xavierleroy

The latter is a rarely-used but cool feature whereas a given structure can be constrained by several signatures to give different views on the implementation, e.g. one with full type abstraction for use by the general public and another with more transparent types for use by “friend” modules.

This is something I’m really interested in, because it seems like a genuine problem that comes up quite often. How hard would it be to take, for example, the Map module in Stdlib, and make an additional Map.Internals module that has full access to the internals of the Map and is also fully compatible with the regular Map interface?