The bar is called a safety squat bar, but is also known as a "yoke bar" or a "spider bar" I believe. It is used for a few reasons:
1. Shoulder injury.
2. Relieving stress upon the shoulder joint when doing either heavy high volume squats or lots of sets of near max squats.
3. To offer a different load placement.
I personally would kill for one of these bars as my shoulder cops a lot of aggravation from squats due to most of my working sets being 90%+.
Want ham and glute development + squatting and deadlifting strength? Box squat bitch!