Being able to change the destination URL is one reason to use a redirect. Many users do not control their tech infrastructure, or have the knowledge to create redirects. This allows them to update or change content without changing their QR codes.
I could buy that, but also the QR and the destination website are usually done by the same web agency.
The issue is that probably the web agency is externalizing itself the QR thing, or try to centralize all their clients in the same system.
