For the plane's wing to be most efficient, it needs to be horizontal (lift force is directly up...). By building in di-hedral, neither wing is optimal when the plane is level, but the moment the plane banks slightly, the lower wing is more efficient than the higher wing, and thus produces more lift. This tends to make the plane more stable, because the plane is "self leveling".

It also makes inverted flight less stable.

Very good idea for trainers, and almost all full scale planes.

gus