According to the owners manual, it has 2 purposes :
1. to unlock and lock the tonneau cover latch during manual top up/down.
2. to remove the lower right side grille for access to the front tow hook.

A thin 10mm wrench will server for #1, a long screwdriver will server for #2.

Interesting thing is : they included this tool with the roadsters, but did NOT provide the 5mm hex wrench which is also necessary for the manual top up/down locking of the rear bow latch.