Why do people always tell you to wear clean underwear in case you get in an accident? Why would people see your underwear if you got in an accident? Are they assuming that you'll hit so hard that you'll fly right out of your pants? There you'll be lying in the street in your skidmark underwear when the paramedics show up. But honestly, would that really look so odd, having skids after an accident? I mean, there's nothing wussy about having the crap scared out of you.
Or maybe it's because the paramedics check that sort of thing while you're unconscious. "You keep him alive, while I rifle through his pockets for change and check his underwear for skids." Then they roll you into the ER, and the attending physician asks for your status. "Well, we've stabilized his breathing, his heartrate's fine, and he's wearing Tweety Bird boxers with skids."
My wife seems to think it has to do with when they're changing you into your hospital gown. But that still doesn't tell me why some nurse is inspecting my underwear to such a close degree. I say that if you're going to look, then you deserve whatever you see.