Belts are not just for holding up your pants. While they do help in holding them up and making sure that the backside of your pants stays even all the way around your waist, they add a finishing touch to your outfit.
The belt provides a clear separation between your torso and your legs as well as adds a nice horizontal line to the top of your pants. If you are tucking your shirt in then belts are a must, but if you are not, then they are just highly recommended. Don't forget to match your belt to the rest of your outfit though. Find this exact look here.