Why do guys wear underwear underneath there swim trunks?

I have no idea why some people do it. I know some people wear trunks like a style around the beach with no intention of hitting the water. Most trunks have a built in mesh lining that holds everything in place anyway.

One is not supposed to wear underwear with swim trunks. They all have a mesh lining to hold your genitals in place.

There is however, a general tendency to want to totally hide one's genitals, even from other males in the showers and changing rooms - this is crazy.

Carry on behaving normally and keep underwear for under trousers - you don't need it under swimwear.

I wear underwear when I don't plan on getting wet. If I'm going to the beach or washing the car, I don't wear underwear.

I wear underwear with swimwear because the ones I usually get dont have bottoms and they also dont come in my size well. That means they can sag when getting out of water a bit and I personally do not like showing cleavage.