Xcode which I should never update!

In Swift the label is by default required for all parameters except the first one in functions. Labels are however required by default for all parameters in constructors (the ones making use of the init keyword).

The fact that for functions it is required for all parameters except the first one comes down to history. In ObjectiveC developers used to encode what the parameters were to contain into the function names.

lengthOfBytesUsingEncoding
stringByReplacingOccurrencesOfStringWithString

This particular function was supposed to receive two parameters, bytes and an encoding. In Swift this same effect can be had using named parameters but it is more clear now as the label shows up next to the parameter instead of right after each other at the end of the function name. I'll give you an example very soon, but notice how the default encourages that the first parameter is still encoded into the function name.

lengthOfBytes(someBytes, using: someEncoding)
stringByReplacingOccurrencesOfString(someString, with: someOtherString)

You can however for your own functions overrule this behavior and you can also change the labels.

externalName internalName: Type

The external name is the label you have to specify when calling the function. If you don't specify an external name it will use the default. If you put _ as external name it means "Don't use a label". Prior to Swift 2.0 you could use # on the left side of the internal name which would duplicate it as external name but that functionality has now been replaced with an error message informing one of the change. Instead one will now have to be explicit about the external name.

func a(height: Int, width: Int) {
}

func b(height: Int, _ width: Int) {
}

func c(tallness height: Int, fatness width: Int) {
}

// No longer works as of Swift 2.0
func d(#height: Int, width: Int) {
}

And you would call them like

a(5, width: 10)  // This is the default
b(5, 10)
c(tallness: 5, fatness: 10)
d(height: 5, width: 10)

Does this answer your question?