[1] Especially for self-driving cars, the exact and continuous knowledge of the navigating object's location is essential.
[2] GPS and other satellite based systems (GLONASS, GALILEO, BEIDOU, QZSS) provide a way to learn one's location, but these methods require free field conditions in order to receive the radio signal.
Various satellite systems are subject to switching-off or reduction of data precision by the company or government that runs them.
In situations where the signal cannot be received reliably, alternate sources of location data are needed.
A hybrid system provides fault tolerance for each underlying method and improves the overall precision of the result.