Maybe the most important, because so many things follow from it. Whether you're running 50 meters or 50 miles, here's what upright running will do for you. You'll move as one. Your muscles won't be wasting energy maintaining balance, correcting for forward lean and so on. You'll run more easily.
Upright posture helps keep the whole body in proper alignment.
When you're in proper alignment, you'll generate maximum power, which reduces the effort required to run at any pace. You'll increase speed more easily. An upright body allows your legs to extend maximally with no extra effort. You'll get injured less. When you reduce inefficient leanings and extraneous motions, your muscles and tendons will spend less time correcting these problems.