Swift left side assignment

tfttere Source
class ViewController: UIViewController {
    @IBOutlet weak var display: UILabel?

    var userIsInTheMiddleOfTyping = false

    @IBAction func touchDigit(sender: UIButton) {
        let digit = sender.currentTitle!
        let textCurrentlyInDisplay = display!.text!
        display!.text = textCurrentlyInDisplay + digit

Stanford Swift calculator course does the last display!.text with no explanation. I understand the previous let textCurrentlyInDisplay = display!.text! but why does the display on left side of the assignment have to have and exclamation point?



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