Ascending is the act of going up a rope. On a thick rope with warm hands, I can climb roughly 30 feet of rope hand-over-hand. On a rappelling rope with freezing cold hands, I can't even get my feet off the ground. In these cases, its best to use either mechanical ascenders or slide-and-grip (friction) knots. Ascenders or friction knots give you the advantage of using the muscles in your legs instead of just your arms. And you also don't fall to your death if you get tired. Even if you don't think you'll need to ascend back up a rope, it's a very good skill to have in case of emergencies.