There’s probably a bunch of reasons for the multi wing design, but the big one is going to be improving lift/carrying capacity without increasing the width.

The most efficient wings for low speeds are glider wings: as long and thin as possible. That makes them inconvenient to pack and folding joints are weak points. The second wing adds lift, but also problems: it’s less efficient than a single wing of the combined length would be and the front wing makes the rear wing less efficient. The winglet improves the situation somewhat. Facing downward also improves maneuverability.