Why we make diapers with two or three waterproof layers?

Why we make diapers with two or three waterproof layers? Is it more expensive to make?

Yes, it is more expensive to make. Double or Triple layered waterproof cloth diaper is more durable, highly water-resistant and breathable. 

Double waterproof layer is available on the Newborn Little Green Baby Bamboo and the One Size Original Squared

Triple waterproof layer is available on the One Size Good Night Heavy Wetter