As developers, more often than not, you have access to fast and reliable internet connection, all day long. But the users you target won't. Limited data plans, unreliable network coverage, commuting : those are real obstacle that your users will see daily. This means that network will only be available at some times, and not necessarily when the user wants to interact with your application.
An introduction to offline first development on Android, given with Jérémy Bartolomeu Bonze