What is the use case for convenience initializer?

In swift there is the concept of designated initializer (which is the "normal" constructor we know from other languages, I assume).
But there is also something called Convenience Initializer, which I do understand how to write, but the point is lost on me.
As, If I understand correctly, you can achieve the same thing without writing the keyword convenience, or not?

Actually it's very easy to understand them: they are initializers with default parameters.

From the docs:

Convenience initializers are secondary, supporting initializers for a class. You can define a convenience initializer to call a designated initializer from the same class as the convenience initializer with some of the designated initializer’s parameters set to default values. You can also define a convenience initializer to create an instance of that class for a specific use case or input value type.

Example:

class A {
    var a: Int
    var b : Int

    init() {
        a = 0
        b = 0
    }

    /*convenience*/ init(a: Int) {
        self.init()
        self.a = a
    }
}

In the above case, you cannot call self.init(), you have mark your initializer with the convenience keyword, otherwise it will be a compiler error. So you cannot chain two designated initializer from the same class.