Stabilizer/Load-lifter/Load-adjuster... Straps
Those are stabilizer straps, also known as load lifter or load adjuster straps. You typically have another set of stabilizer straps on your waist belt as well. These straps essentially prevent your bag from flopping around on your back and help balance the load, which will ultimately lift the weight of the bag off of your shoulders and better secure the bag to your back. If you really want to get a sense of how beneficial they are, try going for a hike with those straps completely slacked off, then compare that to hiking with them properly adjusted.
Your bag must be properly fitted and packed* for the straps to be fully effective. Adjust the back length on your bag (if it is adjustable) so that the shoulder straps drop about 2" (5cm) behind your back with the hip belt comfortably fastened around your hips. Then adjust the buckles on your shoulders for the stabilizer straps so that they are at about a 45° angle.
stabilizer straps http://ems.imageg.net/graphics/corp/load-lifter-straps.jpg
*You want to balance the weight in your bag so that it will lean more "towards" your body than away from your body. One way to test if you've properly packed your bag (If your bag has a flat bottom), it to rest it on the ground to see how the load is distributed. If the bag sits upright or falls forward, then you have properly distributed your load. If it falls backward, then your bag is going to force you to lean forward more to balance, which will pull more on your shoulders and ultimately fatigue you faster, in which case it would be beneficial to repack your bag.