From Gun Noob:

A spotting scope allows you to easily examine your target from the comfort of your own shooting stand. It has advantages over using your rifle’s scope in that it gives you a bigger and clearer picture of your target. It also, by the nature of its optics, powers through the mirage effect so you can see your target better on hot days and over longer distances. Ultimately, the purpose of a spotting scope is to allow you to evaluate your shooting easier than with the scope on your rifle which is meant solely for targeting. While a rifle scope can be used in a pinch, practice on a range isn’t one of those “in a pinch” times. If you’re not using a spotting scope, you’re making things harder for yourself.