Why do we have to add "@objc" in front of "func" in Swift 4 when dealing with target/action selectors?

Question

Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(test), userInfo: nil, repeats: true)

In Swift 3, this is the code

func test() { }

but in Swift 4 and Xcode 9 you have to write it like this:

@objc func test() { }

Why is that?