The offline tour map is scaled to online maps and offers more detailed information than the online maps, along with the offline map bring you a peace of mind when you know where you are when internet is not available and you are in/outside the GWMP parks!
In case get lost, you can plot your location latitude/longitude on the offline maps and notify people who are trying locate you.
You can do lots more with the app such as recording then sharing your trips as named trails with friends, displaying your friends locations, contact sharing, note keeping, timer, fitness etc.
The large app file size is due to the embedded offline tiles that you don't have to download yourself.
With Family Sharing set up, up to six family members can use this app.