There's no such thing as "the value IO ()". The type IO () has tons of possible values, which all have different internal representations:

1) The IO action that does nothing.

2) The IO action that prints the string "Hello World" and then returns nothing.

3) The IO action that reads a string from the console, reverses it, prints it back, and then returns nothing.

4) ...

It's a common misconception to think that IO X is a wrapper around X with some "type system magic" to prevent it from being used outside IO. It's nothing like that. For example, IO String is not a wrapper around a String, doesn't contain any String inside, and cannot be converted to String. Meditate on the fact that System.IO.getLine is not a function, but a value of type IO String. Then you will understand why IO () has tons of possible values.

You are asking the right questions, it just means you're ready to go one step down the rabbit hole. You need to carefully, and slowly, read SPJ's Tackling the Awkward Squad. You need to allow yourself to skim over the parts that are perplexing on first-reading and take them on faith. With more understanding you'll find even the answer given there not particularly satisfactory (in fact the paper points it out explicitly, trust me you'll know when you're in the precise section).

At this point you'll either accept the state of affairs, or want to go deeper down the rabbit hole, at which point you become more an academic than a programmer per se.

In a very precise sense what groovy2shoes is saying is exactly right, and you need to understand that truly strange weirdos inhabit IO () whereas, as you point out, only one thing, modulo non-termination, inhabits ().

"The only conceivable value that the type IO () can have is still the value IO ()."

There is some magic around the IO type, but it's not needed to answer your question.

If you look in GHC.Types in the package ghc-prim, you can find the definition of IO:

    newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

An IO action is a function from the state of the world, to the state of the world plus a value. So the compiler may know it's giving you (), but it doesn't know what the new state-of-the-world will be.