I can say, if we are speaking about the contemporary worldtimers:
- the 240 movement
- the ability to read the time of the 24 timezones at the same time
- easy to operate, easy to understand
- a clever complication by Louis Cottier
- an iconic design coming from the original watch
- the feeling to travel in your head
...
- the charm of the watch:

To understand how it works:
the city of reference is located at 12 o'clock (here Paris). It is 4:05 pm in Paris Thanks to the cities and hours independant rings, I can read the time everywhere in the world (7:05 am in LA). The pusher is used to change the city of reference.
Hope it helps.
Fx