Why is this the answer?

The answer is thumb because this is a very clever riddle that uses tricky clues to describe one of the most important parts of your hand! You have a thumb on your left hand and a thumb on your right hand, which is what the first line means by "Used left or right." The middle part is the trickiest to understand. It says, "I get to travel over cobblestone or gravel." Imagine you are holding a long walking stick or a shepherd’s staff. Your hand, with your thumb, holds the top of that stick. Even though the stick itself is what touches and "travels over" the rough ground, like cobblestone and gravel, your thumb is right there helping you grip it for the whole journey. This shows how helpful the thumb is when you are traveling over different kinds of ground. Your thumb helps your hand stay strong as you walk, moving along with you the whole time. The last two lines give away the answer the best. The riddle says, "Used up, I vie for sweet success, used down, I cause men great duress." This is a perfect description of the famous hand signals you make with your thumb. When you point your thumb up, that is the thumbs up sign! It means "good job," "yes," or "sweet success." You are showing that you approve of something or that everything is going really well for someone. But if you point your thumb down, that is the thumbs down sign. The word "duress" is a way of saying worry or sadness. A thumbs down is a symbol for something bad or disappointing. The people who saw a thumbs down felt worry and stress because it meant something failed or was bad. Long ago, this gesture was even used in coliseums to decide if a gladiator would live or die, which would certainly cause "great duress"! So, whether you are holding a stick or giving a signal, your helpful thumb is the key to solving this wonderful word puzzle.