when working gps gives excellent position.
when working that is.Nothing is perfect or guaranteed.In the vast majority of cases it is fantastic but...
aircraft must have some kind of dead reckoning / independent nav device.
Mixing provides cross checking and constant updating..If a solar flare took out some satellites or effected their ephemiral data or more likely a lightning strike could blow both GPS antennas.Seen that myself.
pre gps it was dme dme vor or similar with irs to fmc.
Checking cross checking.......To be sure to be sure...