[Haskell-beginners] recursive 'let' ?

I understand that the definitions introduced by 'let' can be recursive, even mutually recursive among several names. Why would you want to do that? I saw contrived examples, and wonder why the authors never show a realistic example. let b = f a c c = f a b in ... I see that makes sense in light of lazy evaluation: b is really an alias for a (recursive) function, not a value that needs to find fixed points. Is this used for common idioms and problem-solving approaches in Haskell? --John