Almost every application needs to store data persistently at some point. Any data that needs to remain intact when your application is stopped must be placed in some kind of data storage system where you can retrieve it again later. You can store all the data on the server, but then your application will only function when the user has an active internet connection and will only ever be as fast as their available connection. You can also store data as files on the device's local filesystem, but this means you need to either load all the data into memory and save the whole application state every time it changes, or you need to write complicated logic to maintain integrity between the various files your application will write.
The Android ecosystem has a large number of database systems available, the most popular of which is probably SQLite. Data can be saved in SQLite tables and retrieved through structured queries. This provides an ideal way to store all of your...