Structure of "if" and "else"

I'm a beginner with Swift and had a quick question about the structure of "if" and "else". I am following a tutorial about validating usernames and passwords.

This is the current code I have:

let username = "brian0306"
let password = "secret"

if (username == "brian0306") {
    println("😍")
    if (password == "secret") {
        println("❤️")
    } else {
        println("Uh oh, your credentials are wrong!")
    }
} else {
        println("Uh oh, your username is wrong!")
}

Why is the tutorial suggesting that I place the password validation code within the username validation function?

Is it more effective if I validate the username and password in separate functions?

Brian