When someone stops me in the hall and asks me to take care of something by a certain time, I press a button, say "Remind me in 45 minutes to do blah blah blah" or when I need to remember something, I go "Remind me when I'm at Lake Road and 5th to stop for milk".
Some of us aren't walking datebooks with an uncanny ability to remember every second of every day.
Also, I can't imagine why anyone would willingly try to drive somewhere at night to someplace they've never been, unable to read unlighted street signs in a residential area, with people going twice the speed limit behind them and not have a nice lady come out of their car speakers saying "turn left."
Not seeing a need for a GPS device or an unpaid, uncomplaining, electronic personal assistant that can recognize what you say and act upon it is, in my opinion, the exact same thing as not seeing a need for a gas station map or a computer with Outlook installed.
I don't know about anyone else, but I prefer things to be easy to do instead of a hassle.