For me it's for traffic. There's about 4 major routes I can take between my home and office, and with different traffic changes (wrecks, construction, conventions, etc) the difference between them can be over an hour.

It's why I can't use Organic Maps despite being a contributor to OSM.

It can tell us when traffic is backed up on a route. I of course know my work route by memory, including 5-6 route changes I could take. Though, why would I take those changes unless I know ahead of time I needed to? Using navigation can tell us problems on the route, whether or not we can go around effectively, or whether or not we should just let someone expecting us earlier how late well probably be.