Step 1: Forming a neater shoulder edge altogether
By using this method you are eliminating the saggy baggy ends that can sometime occur. This method would be especially useful for the inexperienced knitter, where the tension is quite often a bit iffy because of inexperience.
Step 2: The end!
Also when you have to knit two identical pieces, for example the sleeves, it is a good idea to knit them both together on the same needles using two separate balls of yarn. The reason for doing this is you will be sure to make them both the exact same length and your increasing and decreasing will be done in the same places. Also (and very important!) if you make a mistake with the decreasing, it's not so critical, as you have made the mistake at the same place on both sleeves.