 The use of a fly is optional, and many styles do not utilize a fly at all.In the case of men's underwear, briefs, unlike boxer shorts, hold the wearer's genitals in a relatively fixed position, which make briefs a popular underwear choice for men who are participating in athletic activities or who feel they need more support than loose-fitting underwear can provide.In addition, boxers often ride up the body when the wearer is running.