[Haskell-beginners] Why ShowS?

Small point I'm confused about: Why is it that in the Numeric module, all the "show" functions return a "ShowS" type (String -> String) instead of just the String? The ShowS type is described as so... "The shows functions return a function that prepends the output String to an existing String. This allows constant-time concatenation of results using function composition." What is meant here by "constant-time concatenation of results using function composition"? Perhaps someone could provide an example? -- frigidcode.com theologia.indicium.us