It is a guarantee to you that the data structure has not been tampered with by the time it reaches you. // pseudo code var fruitBasket = new FruitBask

It is a guarantee to you that the data structure has not been tampered with by the time it reaches you. // pseudo code var fruitBasket = new FruitBasket('banana, 'apple) muckWithFruits(fruitBasket) suspiciousLogger(fruitBasket) weirdCallback(fruitBasket) // here assert(fruitBasket.length == 2) By the time we reach "here", we know that none of the functions since the inception of fruit basket have tampered with it. This may not be a big deal if you are disciplined about never modifying your source data, but this moves the guarantee from a soft one to a mechanical one. And then there's all this typical stuff about functional programming blah blah... But that's the most practical example I can think of.