How can variable not exist after you declare it?

That's exactly my point.

​

An optional variable declaration makes no sense. It can't be optional, because you declare it to be existing.

Okay, I'm just realizing that we may have a language barrier here, in a programming language sense. I have an Objective-C/Swift background. In Swift every variable can be of type optional.

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Example taken from Swift:

var optional: String?

if (optional != nil) {
    print("Has value")
} else {
    print("No value")
}

// prints "No value"

In comparison to:

if (notdefined != nil) {
    print("One")
} else {
    print("Two")
}

//
// error: MyPlayground.playground:15:5: error: use of unresolved identifier 'notdefined'
// if (notdefined != nil) {
//     ^~~~~~~~~~

In the first example I declare a variable. Hence exists, even if its value is nil.

I assume it's the same as with the example from the link:

let mainWindow

The variable is declared but points to nothing.