The "old" purpose of multiple threads is indeed to do slow IO operations in the background. But nowadays, given that all processors have multiple cores/threads (the machine I'm writing this on can handle 24 threads in parallel, for example), it also very common to use multiple threads for calculations.
That said, it's not easy to use multiple threads for physics, because the result of one thread's calculation can have an impact on the other. Imagine 8 boxes colliding with each other. If you calculate each box in it's own thread, the outcome of one would depend on the collisions of all the others, which is almost impossible to synchronize properly.
jPCT(-AE) itself isn't thread safe (but you could do calculations on independant objects or tasks in multiple threads) and neither is OpenGL(ES). If you want to use multi-threading in one way or the other, you have to make sure that the tasks that are processed by the different threads are independant (for example: One thread for physics, one for sound, one for path finding...).