It is to be used to bottle from a conditioned keg. I would see it being used (in my brewery anyway) to bottle for a party, wedding, to bottle the last 5/10 litres of a keg to free up space in a kegerator, entering competitions with beer in top condition, gifting beer etc....keeping O2 out of the bottle to help stop spoilage when storing the beer for a longer period of time.
I did some trials on bottling from the tap and even with capping on foam the beer degraded over a 2 week period compared to what was in the keg. The gun should stop this from happening.