The Nuva Symbols contain the Toa Nuva's power. If they fall in the hands of someone that does not respect the Toa, the Toa will lose their powers. The Bohrok-Kal exploited this in an attempt to free the Bahrag. The Toa Nuva also had a mental connection to the symbols, allowing them to pour their energy into the Bohrok-Kal, thus overcharging them.
The Nuva Symbols are the keys for the Nuva Cube, a lock to the Bahrag's prison.
The Toa Nuva likely took their symbols with them when they evacuated the Great Spirit's body