Imagine (like you should fairly regularly) that it's the end of the world and you need above average survival skills to persist in the every man/woman for himself/herself post-apocolypse wilds.
Well, loud footsteps (otherwise known as "stomping") are often an indicator of braking: where your joints are not flexing to carry the impact of each stride into the next one, but instead are redirecting forward momentum into shockwaves that move up through your body and down into the ground. And you don't want to brake (or even break)! If you land your foot in front of your knee (like the ninja in the picture above) some of that landing force will counter some of your forward momentum.
If you or your running partners can hear your footsteps clearly, you'd probably starve in our scenario - or more likely, you'd be food for something else.
By running more silently, you can increase your speed and efficiency. Particularly on hard surfaces, quieter running requires less contact to the ground, and that is one idea most experts seem to agree on, that increases speed, efficiency and reduces injury.