The reason why you use a biplane design is to get more lift per unit span, so you can keep wingspan within some arbitrary constraint. As with all engineering,

The reason why you use a biplane design is to get more lift per unit span, so you can keep wingspan within some arbitrary constraint. As with all engineering, there is a tradeoff involved, and with biplanes you don't get double the lift because of the boundary layers of the wings interacting. You also pay a drag penalty as you have twice as much skin area exposed to airflow, and it complicates the design because in the transonic région you can get interactions between shockwaves causing flutter and other not so fun effects.