In CoffeeScript, what is the existential operator for and how do you use it?

In CoffeeScript, what is the existential operator for and how do you use it?

I see all sorts of questions about the existential operator on SO, but
none of them ask the fundamental question of "what is it for" and "how do
you use it?" so I thought I'd ask this here.
The answer here would probably suffice as an answer to this question, but
the problem is the question title doesn't suggest that. As a result, it's
very hard to find this question by a google search. So, my intention here
is to make it easier to learn what this operator does from a google
search.
BTW, I am aware of the section in the The Little Book on CoffeeScript
titled "Aliases & the Existential Operator", but for some reason I don't
like its explanation. It doesn't make me feel like "I get it".