It depends largely on the satnav. London is such a complex city with a lot of nooks and crannies that people don't really understand and that satnav may or may not know about. Black cab drivers are also privy to the experience of knowing what places are busy when, and why, which satnav tools often can do too but not necessarily to the same degree of accuracy as a black cab driver whose job is to think on their feet and react to changes in the environment based on what they know and what they observe.

With a minicab driver simply following instructions from a satnav, you might as well just have a self-driving car, which is no bad thing.

With a black cab driver using their knowledge and experience and eschewing the instructions from a system which wasn't designed specifically for London, you may have a more reliably fast journey.

Plus, they could even give tourists some interesting facts about the places they're driving through!