What's the purpose of optionals with Swift?

Out of Apple's Swift Programming Guide:

Here’s an example of how optionals can be used to cope with the absence of a value. Swift’s String type has a method called toInt, which tries to convert a String value into an Int value. However, not every string can be converted into an integer. The string "123" can be converted into the numeric value 123, but the string "hello, world" does not have an obvious numeric value to convert to.

The example below uses the toInt method to try to convert a String into an Int:

let possibleNumber = "123" let convertedNumber = possibleNumber.toInt() // convertedNumber is inferred to be of type "Int?", or "optional Int"

Because the toInt method might fail, it returns an optional Int, rather than an Int. An optional Int is written as Int?, not Int. The question mark indicates that the value it contains is optional, meaning that it might contain some Int value, or it might contain no value at all. (It can’t contain anything else, such as a Bool value or a String value. It’s either an Int, or it’s nothing at all.)