Okay: The Kal want the symbols to take the Toa's powers away. This does that, because they got their powers in the Bahrag prison. Okay, so that part didn't make much sense, but listen to this:
If the symbols are put together, they unleash the cube, to free the Bahrag and cause them to rule once again! The Kal were probably Failsafe Devices just in case the removing of obstacles failed!