It is nice to know when there are traffic incidents ahead and sometimes it'll route around it for you. I like knowing my ETA also
Some people drive to work.
Some people don't need a navigation system for a route they drive every day.
I tend to use Google maps anytime I drive in Seattle - they've added information about accidents and closures that I'd otherwise miss. When I did drive to work it'd routinely save me time when there was a wreck on i-5 by telling me that and giving me another route.
It can recommend an alternate route if there is traffic on your regular route.