At one time, it was an important item of European clothing in the 15th and 16th centuries.
At first, the codpiece was entirely a practical matter of modesty. Men's hose were typically very snug on the legs and open at the crotch, with the genitalia simply hanging loose under the doublet. A shortening of the doublet resulted in often-exposed genitalia, so the codpiece came into being (there are other versions of the origin of the codpiece).
As time passed, codpieces were shaped to emphasize the male genitalia and eventually often became padded and bizarrely shaped. They also often doubled as pockets, handy carrying places for a variety of items.
The lead singer of 80s music group Cameo famously wore a large, bright-red codpiece in all of his performances. Also, Alex and his gang members from Anthony Burgess' book A Clockwork Orange (and later Stanley Kubrick's movie adaptation) wear codpieces.
