Target rifles are shot prone, with a s;ling to make rifle steady. 3 shooters would shoot at the same target, taking turns. In a 15 or 20 shot match, you would be holding the rifle for a long time. The rest was used to hold the rifle, between shots.
You did not take your arm out of the sling, but just swung the rifle over a bit and rested it in the rest, or had the rest closer, and rested the back of your hand or arm on it, between shots.