Clearly you can see in the implementation of the function that it doesn't rely on any globals. However if you look at the function as a black box, you cant really be sure what global state it relies on. This could be an issue if you use larger libraries you didn't write yourself (or in my case, if I use code I wrote more than two weeks ago!).
In a language like Haskell the type signatures clearly indicates which global state the function (and other functions called by it) has access to, which makes it a lot easier to reason about side effects, even for code where you haven't read the source.
Of course, this approach also have its downsides. If you decide for debugging purposes to add a logging function to an otherwise pure function deep inside a pure part of your program, you may have to change a whole lot of code.
In a language like Haskell the type signatures clearly indicates which global state the function (and other functions called by it) has access to, which makes it a lot easier to reason about side effects, even for code where you haven't read the source.
Of course, this approach also have its downsides. If you decide for debugging purposes to add a logging function to an otherwise pure function deep inside a pure part of your program, you may have to change a whole lot of code.