A prime example is this: You and your partner are climbing at a dream multi-pitch sport crag. The rock is steep in some places. So the leader goes up, as usual. Then the second climber follows, picking up draws along the way. Maybe some sections are a little run out, and all of a sudden...POP! The second climber peels off and is hanging in space. Just swinging around, unable to reach the rock in order to get back on the climb. Ever been there? Try it some time, it ain't fun.
Never leave the ground without 2 pieces of cord or webbing if you'll be climbing multi-pitch routes. These two lightweight pieces will be a god-send in this and other situations. Each piece can be anywhere from 2 to 3 feet long (looped), the diameter of the cord should be 5-6mm and the webbing about 9/16-11/16 inches. It all varies depending on the rope, conditions, and the webbing or cord you buy. It's cheap to buy, so experiment with what works best for you before you leave the ground.
The three knots below are easy to set up and are designed for ascending the rope because they can be loosened as you push them up, but they bind tightly when pulled downward. The reason you need two is to make one ascender for your waist and one ascender for your foot. As you hang on your waist ascender, you raise the foot ascender. Then as you stand up on the foot ascender, you raise the waist ascender. Then hang, raise the foot ascender, stand up, raise waist ascender, and so on. The goal is to get to the next draw, where you will then clip in directly, remove your ascenders and have your belayer take up the slack. Then unclip and continue to climb.
All of these knots can be used for various tasks such as hauling and self-rescue. Every climber should know how to tie at least one and understand how it can be used. I suggest hanging a line in a gym somewhere and practicing ascending a rope with of these knots. Knots can be considered every climber's connection to their life line.The Good Old Prusik
The Bachman Knot
The Bachman is fun because it has uses a carabiner to create a handle for you. Oval carabiners work the best. And don't pull down on carabiner, use it to push the knot up.