They both share a similar purpose, which is to track and display the time in another time zone. The difference is that a world timer displays multiple time zones simultaneously, whereas a dual timer indicates just two time zones.
For people who have business or personal connections around the globe, a world timer is a nice option because it tells you quickly what time it is where they are, so you can avoid doing the mental math to figure it out.
Quite a few brands offer this handy complication, and most take similar approaches by showing all 24 major time zones on the dial simultaneously. Many feature a rotating ring numbered from 1 to 24. When the ring turns, the numbers line up with a list of 24 cities to indicate the hour in each location.