Being looped up in a sling takes the muscle tension out of your bicep. That's an advantage for all positons that have your support elbow/upper arm resting on something, such as prone, kneeling, and sitting.
It doesn't really help as much for the standing position. There still won't be any muscle tension in your bicep but you will notice some in the frontal deltoid that wasn't there in one of the supported positions. It might help in the standing if your body configuration is such that you can lean back and rest your elbow on your hip. Then your deltoid and bicep both should be relaxed. Not everybody seems to be able to achieve this. I'm pretty steady sitting and can't shoot for squat standing!