Why do we always initialize, and is it necessary?

The first example in the Swift documentation you provide a link for creates a base class, Vehicle, with two stored properties: currentSpeed and description.

class Vehicle {
    var currentSpeed = 0.0
    var description: String {
        return "traveling at \(currentSpeed) miles per hour"
    }
    func makeNoise() {
        // do nothing - an arbitrary vehicle doesn't necessarily make a noise
    }
}

Next, a subclass is created:

class Bicycle: Vehicle {
    var hasBasket = false
}

And a Bicycle object:

let bicycle = Bicycle()

Whose currentSpeed is updated as follows:

bicycle.currentSpeed = 15.0

Your question is whether there is any initialization going on here. Answer: yes. When the bicycle object is created its stored properties (instance variables in other languages) are initialized. Apple defines initialization, in part, as the process of making sure that every stored property is given an initial value; something that has to happen before an object can be created. When the bicycle object is created it inherits the stored properties of the Vehicle class, and uses the initializer of that superclass to give them their initial values. In particular, currentSpeed is set to 0.0.

In this case the initializer is the default initializer. If you were to write it out, it would look something like this:

    init(currentSpeed: Double) {
        self.currentSpeed = currentSpeed
    }

But since it's the default initializer it doesn't need to be added to the Vehicle class. It's already there, behind the scenes, as it were.

So to answer your initial question, we initialize because Swift requires it. "All stored properties must have initial values. " And yes, it is necessary. They do use initialization in the Inheritance documentation, they just don't illustrate it or talk about it, probably because Initialization is the next chapter.

Happy coding!