it means you can run up behind someone without being heard but can still be seen if someone is looking in your direction. you can if noone else is around stand run at someones back and pull off a sneak attack critical without having to crouch and move slowly.
While sneaking, you have a higher chance to be heard moving fast than moving slow. Silent Running will lower the chance.
It is not a useless perk, you just need to know to sneak (like not being in front of someone you are trying to pickpocket or kill or whatever).
Silent Running means that the speed of you movement is not factored into your ability to stay hidden. The fact that you are not sitting still will still be factored in. You are still less likely to be detected if you are stationary.
I believe the OP was under the impression that silent running gave regular running a sneak-like state where your movement wasn't detected, as it sounds like he/she's comparing crouching and not crouching.