A QR code is useful when someone is standing in front of something and you want them to open a link without typing. That could be a review page at checkout, a Wi-Fi login in a waiting room, a maintenance form on equipment, or a payment page on an invoice.
Keep the destination simple and mobile-friendly. If the QR code points to a long form, make sure the form works well on a phone. Always test the code from the printed size and distance people will actually use.
Use QR Code Generator for URLs, plain text, contact details, Wi-Fi credentials, or short instructions.