Originally Posted by Luca
Because Google isn't very good at picking up on local factors that affect travel speed, such as traffic density, construction, and road closures/re-openings. It'll also always pick the route that is fastest according to its calculations, which is sometimes significantly longer in terms of distance. I've had Google suggest routes that were 1 minute faster and over a mile longer (or more). In reality I find the shorter routes tend to be just as fast, but it depends on really minor things like whether you get lucky hitting the lights or not.
It also won't advise you to take a parallel side street instead of a light-filled main drag in order to avoid traffic lights, or show you how to bypass busy intersections and left turns across busy streets.
But if you know the area well enough to know which parallel side streets have less traffic lights, why would you need directions at all?
I'm not saying I always follow Google's maps to the dot, but I usually don't change the route beforehand, unless something is obviously wrong. If I think/know a parallel street would be faster, I'll take it, but it's not like that's going to totally mess up my sense of direction or anything.