Offline mode allows you to make sure all map-related features are used as if you were online, preventing lots of additional (mobile) data usage. This includes the visible maps, but also search and route calculation. Other convenient features, such as the synchronisation with your MyRoute-app account, keep working without any limitations.
The main difference with airplane mode is that in airplane mode, all network communication is blocked, which means you will also lose other online features (mainly synchronisation) and detecting your location may take longer due to less sensors being available.
Let me know if you have any further questions.