In addition to sibling's comments about jamming and self driving safety, there are many driving situations where there is no or poor GPS reception: tunnels, double deck bridges, double deck freeways, underpasses, urban canyons, actual canyons, etc. Also regional problems. The GPS constellation is in a 55° inclination, so if you are north of ~55N, or south of ~55S, you need a clear view of the southern/northern sky, respectfully, for reception, since there will be no overhead satellites.