Why is lazy evaluation useful?

Question

I have long been wondering why lazy evaluation is useful. I have yet to have anyone explain to me in a way that makes sense; mostly it ends up boiling down to \"trust me\".<

Answer 1

When you turn on your computer and Windows refrains from opening every single directory on your hard drive in Windows Explorer and refrains from launching every single program installed on your computer, until you indicate that a certain directory is needed or a certain program is needed, that is "lazy" evaluation.

"Lazy" evaluation is performing operations when and as they are needed. It is useful when it is a feature of a programming language or library because it is generally harder to implement lazy evaluation on your own than simply to precalculate everything up front.