A dynamic URL QR code encodes a shortened URL (an intermediary URL) rather than the final destination URL. This means the QR code itself never changes, yet the page it leads to can be updated at any time through the URL shortening service's dashboard.
With a static QR code, the encoded URL is permanent. If you need to point it somewhere else, you must regenerate the QR code and reprint every piece of material that carries it. A dynamic QR code avoids that reprint when the service you use lets the redirect target be changed after the fact - update the target and every already-printed code points to the new page.
Use cases are plentiful. A restaurant can rotate seasonal menu pages behind a single QR code on every table. An event organizer can switch from a pre-event landing page to a post-event archive. A product manufacturer can refresh campaign pages on packaging without recalling stock. In each scenario, the printed material outlasts any single link destination.
The cost savings scale with volume. If a batch of 10,000 flyers contains a QR code pointing to the wrong page, a static code means reprinting the entire run. A dynamic code means a 30-second edit in a dashboard. For large-scale campaigns the difference can be tens of thousands of dollars.
Technically, this is the ordinary redirect mechanism of a short URL put to a different use. The QR code carries the short URL rather than the destination itself, and the printed code follows wherever that short URL is pointed later on. Whether a short URL can be repointed after it has been created differs from service to service, so it is worth checking before anything goes to print. Some services also let you count scans alongside it, though what they measure varies as well.