How do I properly use a timer to stagger object creation?

class ViewController: UIViewController {

let manImage = UIImage(named: "man.png")

let buttons = (0..<5).map({_ in UIButton()})

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    createButtons()
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

func createButtons() {
    var pos = 25
    for index in 0...4 {
        delay(Double(arc4random_uniform(5)) + 5) {
            self.buttons[index].setBackgroundImage(self.manImage, forState: .Normal)
            self.buttons[index].translatesAutoresizingMaskIntoConstraints = false
            self.view.addSubview(self.buttons[index])
            self.view.addConstraint(NSLayoutConstraint(
                item: self.buttons[index],
                attribute: .Leading,
                relatedBy: .Equal,
                toItem: self.view,
                attribute: .Leading,
                multiplier: 1,
                constant: CGFloat(pos)))
            pos += 10
            }

    }
}

func delay(delay:Double, closure:()->()) {
    dispatch_after(
        dispatch_time(
            DISPATCH_TIME_NOW,
            Int64(delay * Double(NSEC_PER_SEC))
        ),
        dispatch_get_main_queue(), closure)
}

}

I've been using a bunch of different time delay sample code from searching, but all code results in some of the buttons to be created at the same time. From the line of code

delay(Double(arc4random_uniform(5)) + 5)

does this not mean to delay at least 5 seconds between each iteration of the for loop? Why does it do this?

Thank you.