This question seems to be prevalent among Rolex watch enthusiasts and collectors. The use of the iconic Mercedes symbol has no connection to Mercedes Benz, or the story of Mercedes Gleitze who admirably swam across the English Channel in 1927 wearing a Rolex Oyster.
The Mercedes symbol is a three pointed star (also known as the Rising Star) with each point representing the three modes of transport: land, air, and sea. Unlike Mercedes Benz, Rolex has yet to confirm this as the meaning behind using this design, although it makes perfect sense that this symbol has been incorporated into the hour hand of the Rolex sports models: Explorer (land), GMT-Master (air), Submariner/Sea-Dweller (sea), etc. The consensus out there amongst the aficionados of the watch world is that the Rising Star symbol not only looks great but also offers great functionality.
As you can see above, its purpose is to hold the lume component of the watch and provide visibility of the hour hand when it is overlapped by the minute hand. This is especially useful on four handed watches like the GMT-Master II or Explorer II.