>The question is, does that do them any good? It might help them in the rare case there's a problem with navigation data, but 99% of the time it's a wasted effort
Yes, very much so. The road layout in London is absolutely insane, because the city was established in AD 43. Finding efficient routes requires a great deal of local knowledge and contextual understanding.
Black cab drivers are extremely good at minimaxing journey time. If they have a choice between a route that always takes 30 minutes or a route that sometimes takes 25 minutes and sometimes takes an hour, they'll usually choose the former.
They're required to memorise both routes and landmarks, so they're phenomenally good at fuzzy matching for destinations. If you ask to go to "that place where all the goths and weirdos hang out", they'll take you straight to Camden Lock. They have an uncanny knack for getting confused tourists back to their hotel, based on the scantest of descriptions in the most broken of English.
Uber has a role to play in London, but it's undoubtedly an inferior option. Black cab drivers are a part of the fabric of London and it'd be a tragedy if they were pushed out of business.
https://www.youtube.com/watch?v=Ws-eyqWHNPs