Please correct me if I'm wrong, but my understanding is that most Haskell code doesn't generate values, but "thunks" that evaluate to values if needed.

When you write

  x = 4 + 5

you aren't setting x to 9, but creating a thunk that evaluates 4 + 5 at runtime. An Integer is a thunk that returns an integer and has no other effects. An IO Integer is one that does some I/O before returning that integer.

As I understand it, the only function that has the power to "do" anything (computation or IO) undernormal circumstances is main, which always has type IO ().