I agree with Gerry that this used to always be done. Were it still the convention, we wouldn't be seeing celebrities' underwear (or worse) on the red carpet. Slips are/were used to keep dresses and skirts from being see-through, to provide coverage with a true wrap dress and for the static issue (although static between a slip and a synthetic fabric can be worse). They also help the dress skim the body in a flattering way. I am a big fan of slips in pretty colors. They are practical, sexy and can be used as nighties, too.