iOS WkWebView - What is the difference in this situation?

I have the source code below.

WkWebview will open the app when the URL scheme has tel, sms, item- or mailto. (item and mailto are our custom scheme)

I wonder that what is the difference between using decisionHandler(.cancle) and decisionHandler(.allow) because It is the same to implement UIApplication.shared.open(URL) in both conditions.

 if scheme == "tel" || scheme == "sms" || scheme == "itms-services" {
            UIApplication.shared.open(destinationUrl)
            decisionHandler(.allow)
            return
        }
        
        else if scheme == "mailto" {
            UIApplication.shared.open(URL(string: "ncsone://mail/write"))
            decisionHandler(.cancel)
            return
        }