I took a look at the Maps.me app earlier this year and found it a good deal at 5 bucks, but today the app is free. Why would anyone need another maps app when they already have Apple and Google Maps? The main reason is that both Apple and Google map apps need an internet connection. Yes, some map data can be cached, but all of your points of interest need the internet in order to appear.
Not so with Maps.Me. Once you load the app, you can download maps for anywhere (including hundreds of points of interest for any corner of the world) while you have an internet connection. When you get to your destination, everything is on your iOS device so you won't be burning up those precious megabytes. Maps.Me has improved quite a bit since I last looked at it, and maps are quite detailed.
Maps.Me is not a navigation app. It will show where you are, and show your destination, but it won't give you turn by turn directions. In general, detail is better than what Apple and Google offer with their online maps, and it is particularly good outside the U.S. in obscure locations other map apps don't cover at all.