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.

It’s pretty unintuitive because we’re not used to dealing with ocean sized bodies of water in day to day life. Part of the explanation is just that the prevailing winds pile all the water in the Pacific up against the coast, causing higher sea levels on the West Coast. The lower salinity of the Pacific also causes lower water density, which translates to higher sea levels.