A function in which the input is needed for the computation is very different to one where it's not needed. I would expect the type signature to reflect this, why would you want it otherwise?

Say you have a function which expects objects of type Foo as an input and which returns objects of type Baz. One day, the function is improved by also accepting the type Bar, i.e. Foo|Bar. So Foo isn't needed for the computation, because Bar is also accepted.

Or you have a function which expects objects of type String as an input. But then you realize that in your case, null values can be handled just like empty strings. So the input type can be relaxed to String|Null.

There's a difference between empty strings and Null values imo.

Just "" != Nothing

If you want to handle empty strings as a input in haskell then you have a function of type `f :: String -> b` and you pattern match on your input?

  f "" = someResult
  f ...

Nothing assumes a proper null in that there is genuinely nothing to work with. Still you can make a function to handle it or use `maybe`?