Comfort: try wearing a pair of trousers comfortably at the natural waist with a belt, cinched tightly so that the pants do not move about.
Appearance: a belt brakes up an outfit, which if one is going for the illusion of height is ruined.
Color: the opportunity to experiment with colourful braces is far more enjoyable than with brown, black or other leathers.
Efficiency: In many ways, braces act as a very efficient simple maching for the holding up of trousers.
Longevity: assuming one does not wear the same pair every day, a collection of braces will last for a considerable amount of time, far more so than a belt will, for the pressure of a buckle will create an unsightly impression.
However, if one's trousers are too large, then braces or a belt will not be enough, and ones trousers would sag under their own weight.