I've found it very useful on more than one occasion, but that's not why it's so important. Rather it's that as a result of their Street View mapping Google has developed an extremely accurate map database of even obscure corners of the world which in turn has allowed them to offer turn-by-turn navigation for those areas.
I didn't know that, but in that case it would make sense if causation was the other way around: that Street View was a by-product of "develop[ing] an extremely accurate map database of even obscure corners of the world".
"We're gonna be driving around all these places anyway. Let's stick a camera on top of the cars when we do it."
