Why "cycling specific?"

On those occasions when I need a bicycle short, like when breaking myself in to a new saddle or when I need something to keep leg warmers from slipping down, I wear a "bicycle specific" liner under whatever shorts I want to wear.

http://www.amazon.com/Pearl-iZUMi-El...ref=pd_sbs_a_1

Otherwise, I ride with normal underwear and whichever outer shorts or trousers I want to wear.