If you start navigation while you are connected, and both your location and your destination are saved in an offline map, you can get started then turn off data and still get directions. While this is a bummer, there is (mostly) a way around it. Notice you're warned that you can't get search or get directions if you're not online. You can also delete old maps, see how much space each map is taking, as well as initiate the saving of a new map. From here, you can open a map, zoom around, and look at it just as if it were a paper map. You'll be taken to your information page in the Maps app, and scrolling down to the bottom will bring you to all the maps you've saved. To access your offline maps manually, just tap the silhouette of a person on your search bar inside Google Maps. Anytime you want to use Google Maps without a data connection, offline maps can help.
Or maybe you have a data cap, and you're spending the weekend in a place where you'll need directions often. I often find myself in places where there is no data service from any carrier (and I'm often lost) and Google Maps mocks me by telling me there is no data connection. There are several good reasons why you would need offline maps.
