If you have ever competed outdoors in a field, 3D or target tournament you can understand the importance of being able to zoom in on the target to see how you are shooting. Although using a telephoto scope on your bow is not allowed in competition, you can use some after-the-shot magnification with binoculars or a spotting scope which can be extremely valuable.
For field and 3D, most archers use binoculars, since they are portable and much easier and quicker to align with the target and can be positioned quickly, which is important since you are changing targets often. The use of a spotting scope would be cumbersome to carry and setup from target to target. Although binoculars are allowed in field and 3D tournament, range finders are usually prohibited.
For target, most archers use a spotting scope (also known as a sport telescope, fieldscope or minocular) because once setup the spotting scope does not need to be changed very often. Although you can use binoculars, the value of a spotting scope over binoculars is the magnification and the angled eye-piece which reduces the need to reposition your shooting stance to check the location of each arrow. The rules allow you to set-up a spotting scope within your shooting line space of 80cm. The challenge is making sure it is set up so it does not interfere with your or your opponent.
If you are new to outdoor competitions, consider investing in some type of magnification, since no matter how good your eyes are, the distances become farther and farther as you get older. Understanding how you are doing during each end, instead of after it can be the difference between standing on the podium or admiring it from a distance.