Unless you're lucky enough to have an exactly glasses-shaped head, the dispenser will adjust the curve of the side-pieces to conform to your head, tilt and curve the ear-pieces to fit flush with your head and snugly, but not too snugly, around your ears, and adjust the nose pads, if there are any, to make sure the glasses sit at the right distance from your eyes and with the optical centres of the lenses in front of your pupils. They'll check to make sure the glasses are distributing weight equally and won't press or rub on the delicate skin on your ears and nose, they'll try to prevent the side-pieces pressing on the sides of your head, which can cause headaches, and try to make sure the glasses aren't going to slide down your nose if it gets sweaty, or fall off your face when you look down or vigorously move your head.
If you don't get your glasses fitted properly, you're less likely to get the best vision, more likely to experience discomfort, and more likely to damage your glasses.
But all of these are a lot more important when you're a full-time glasses-wearer with heavy lenses which are sensitive to exactly where they are in relation to your eye. They're not so important if you have light thin lenses in a mild prescription that you don't need to wear all the time (and which might not cost much to replace).