We use nylon (also known as Polyamide) because it's the superb fabric choice for swimwear as it's a "tough" material that can withstand the heavy use of constant body movements. It is resistant to shrinkage, which of course is a must for apparel we use in water most, if not all the time.
Moisture Absorbency: Nylon has low moisture absorbency, so you won't feel the swimwear weighing on you after your dip. Also, Nylon has much less (some articles say zero) absorbency than Polyester. It also means that Nylon-made swimwear dries quicker than a Polyester-made one.
Comfort Stretch: Nylon has a better stretch than Polyester, so it gives a "less tightness" feel on the body, and the swimwear is easier to put on (than a Polyester made one). More stretch may also mean that the swimwear will lose its shape quicker than the one with less stretch.
Chlorine Resistance: Nylon has good resistance to chlorine. That means your swimwear will have less fade in its colours over time if it is more chlorine resistant.
Sun Exposure Resistance: Nylon has good sun exposure resistance. With this property, you worry less about your swimwear thinning out with use.