A QR code usually has a static URL encoded to which the user is redirected after he has scanned the QR code. In case the target URL is no longer up-to-date or does not exist anymore, you may use a dynamic QR Code instead.
With a dynamic QR code, you can change the target URL subsequently. Even if the QR code has already been printed on posters, flyers or in magazines, you can redirect the user after the scan to any landing page, anytime, without having to print the QR code again.
For instance, if you are using a QR Code to redirect the user to a temporary available voucher, you can change the target URL after the promotion has ended.