Purpose

Swim briefs are made for fast competitive swimming with minimal drag. However, for sheer speed some of the full body lycra swimsuits can make you go faster.

As they have no significant UV sun protection you better use them only in indoor pools. For outdoor use you can add a swim shirt. A matching swim cap is often used by lifesavers or waterball players.

Function

Apart from use for indoor pools, swim briefs are useful as aquatic underwear, part of a more comprehensive swimming outfit. Most lycra swimwear won't need a brief underneath, but some waterwear will fit more comfortably.