No tire and wheel assembly is perfectly even in weight. The difference is tiny - often a fraction of an ounce - but at 65 mph a wheel turns around 800 times a minute, and that small imbalance becomes a vibration you feel through the steering wheel or the seat.
Balancing puts each wheel on a machine that spins it and measures exactly where the heavy spot is. We then fit small counterweights to cancel it out.
Beyond comfort, an unbalanced wheel hammers away at your suspension, wheel bearings and the tread itself, which is why the problem tends to get worse rather than settle down.