Haskell has the convention that uppercase identifier is a type constructor, so Incr is actually mean to be used at type level (function & pattern match at type level!), while the incr you defined is not a type level function in OCaml.
The rationale of type level function is that the GHC type checker can use this to reduce usually very complex type expressions, and its usage is always involved with GADT.
While gaga is right, “a clever use” probably means writing human incomprehensible type expressions, abusing the type checker as a theorem prover, which I’m personally against.
Playing with type expressions usually won’t make the programming language having more expressive power (in terms of Turing completeness), it only extends the expressiveness of type system.