Implicitly unwrapped optionals
<p>I'm curious why the implicitly unwrapped optional, <strong>display.text</strong> needs to be unwrapped in the code below.  (From Stanford CS193p)</p>

<pre><code>@IBOutlet weak var display: UILabel!

@IBAction func appendDigit(sender: UIButton) {
        if let digit = sender.currentTitle {
            if userIsTypingNumber {
                display.text = display.text! + digit
            } else {
                display.text = digit
            }
        }
}
</code></pre>

<p>Why is it considered an optional string after the equal sign, but not before?</p>