I’ve never used this app before (I’m downloading it now), but it gets good reviews. It lets you create itineraries, save and organize lists, and share with others. The main advantage seems to be accessing all your maps offline and using it to pinpoint your current location without using data (apparently it can even use locked wifi signals), useful when coming out of train stations, etc. Google Maps can do some of this, but not in every country.
Sounds like it’s best for walking and city siteseeing, not a great option if you’re driving or need navigation.