The Phrase Finder explains that the term has evolved to mean “attractive additional features or fittings,” and is often used in sales pitches for computers, cars, etc. The expression “appeared many times in the 18^th and 19^th century in literal references to warnings or promotional events. These contexts included citations about fire engines, the Salvation Army, circuses; anyone in fact that was trying to draw attention to themselves might do so using a bell or a whistle.” An interesting possibility about how it came to achieve the current definition is that the English cartoonist Rowland Emett designed “Chitty Chitty Bang Bang” in 1968. That was about the time that the phase began being used in America for the current meaning.