[Haskell-cafe] Pattern matching articles/explanations

So what I noticed that "A Gentle Introduction to Haskell" mentioned that wild-cards are useful in constructors. For example: head (x:_) = x So, does that offer any performance benefits over: head (x:xs) = x Or is it primarily to indicate to the coder that xs is useless? I get the impression it has a very similar meaning to the irrefutable pattern in regards to not evaluating it when the function is called. Or am I way off?