You can do custom gameplas and playbooks online?
I like offline because you don't have to worry about being disconnected from the server and losing credit for your game in progress.
I also like that you can save when you like, rather than being subject to the online auto-save. if something weird happens in FA or the draft or you make some random mistake, you're not stuck with it offline, but in online, everything you do is permanent as soon as you do it.
Then there's the issue of edited rosters. Offline CCM allows you to play with the most current rosters, plus you can edit them however you want before starting the CCM.