When and why do we use self?

it is not necessary to use self most of the time. It is necessary, however, when you have a parameter or other variable with the same name as the property. you must use self to distinguish the property from the parameter:

init(firstName: String, lastName: String) {
self.firstName = firstName
self.lastName = lastName
}

you can see that it would not really work if you did not use self

init(firstName: String, lastName: String) {
//is this referring to the property or the parameter?
firstName = firstName
lastName = lastName
}

from the Swift eBook:

In practice, you don’t need to write self in your code very often. If you don’t explicitly write self, Swift assumes that you are referring to a property or method of the current instance whenever you use a known property or method name within a method

The main exception to this rule occurs when a parameter name for an instance method has the same name as a property of that instance. In this situation, the parameter name takes precedence, and it becomes necessary to refer to the property in a more qualified way. You use the self property to distinguish between the parameter name and the property name.

really the only time you need to use self is when you have to clear up an ambiguity like the one above, or when you are referencing properties inside closures.

most of the time the parameters of the init method use the same name of the properties, so you will self used a lot in initializers for this reason.