Underwear offers a barrier that keeps residual urine and feces particals from absorbing directly into your pants.
People that dont wear underwear let their bodily fluids make direct contact with their pants, and then expose them directly to any surfaces their bits might touch.
Not wearing underwear is unhygienic.
Would trousers be the proper word?
If you don’t wear pants, your wee and your shite get on your trousers?
Yes, that is how a Brit would say it