Or overall less noise, especially in smaller cases where you can't fit an extremely large (read: large fan) direct attached air cooler. Larger fans mean more cooling per dB.

Also, water coolers have a lot more thermal mass (because, water), which means that the fan noise that remains can be less annoying as it doesn't ramp up and down as fast with CPU usage, assuming you're driving your fan curve from water temp and not CPU temp.

So it's not fair to say they're just about "showing off" IMHO. There's actual quality of life benefits if you care about noise.