They are used in competition however I could see them being useful for duck hunters too. But you mount it to your semi auto shotgun on the fore end just ahead of your ejection port and keep an 1 extra shell in it.
When your gun runs empty, it just takes a second to swipe your hand over top, catch the shell, drop it into the ejection port and hit the bolt release.
You can fire one more round or load more in the mag tube.
Match savers have saved me many times.
Anxiousness is the axeman of accuracy.
IPSC and 3gun.
You drill the forstock and use the provided hardware.
https://www.youtube.com/watch?v=EtEjfByPJ3s