Pokemon go uses the google maps api to create the map you are using, however it will redownload the map everytime you use it. By pre downloading the map via wifi you safe a lot of data in your data plan and reduce battery usage of the pokemon go app significantly.
To download an area to use offline, follow the steps below.
Tip: The biggest size for an offline area is 120,000 square kilometers. If you try to save a bigger area, you'll be asked to zoom in to a smaller area.