What's the reason of 'let rec' for impure functional language OCaml?

Question

In the book Real World OCaml, the authors put why OCaml uses let rec for defining recursive functions.

OCaml distinguishes between nonrecurs

Answer 1

I think this has nothing to do with being purely functional, it is just a design decision that in Haskell you are not allowed to do

let a = 0;;
let a = a + 1;;

whereas you can do it in Caml.

In Haskell this code won't work because let a = a + 1 is interpreted as a recursive definition and will not terminate. In Haskell you don't have to specify that a definition is recursive simply because you can't create a non-recursive one (so the keyword rec is everywhere but is not written).