I purchased one from Freeland's about 40 years ago for 4 position competitive shooting. It was/is called a "cuff sling". In use, the cuff is positioned on the arm and secured. The length to the sling attachment point can then be adjusted with the buckle/strap arrangement when changing position.
It was generally used in prone, sitting and kneeling positions. Many competitors would mark the location on the belt for quick position changes. As HiBC stated, "the shooter can easily disconnect from the rifle". The sling, in conjunction with a hand stop could be adjusted to give a rock solid shooting position. IIRC I paid about $50 for a new left hand model...
It appears they are still being sold going by the link the OP posted. Definitely a good thing to have if you engage in 4 position target shooting.