I didn't know that offline access was possible. On the rare occasions that I want to analyze my collection outside the game, I export it to a CSV.
Well, it was simply when you launched the client, you could go to the collection screen without signing in.
For somebody like me, who travels a great deal and does not always have internet when doing so (either en route and occasionally at the destination), being able to build decks offline is critical to being able to play when I can get online. Otherwise I need to spend a lot of my sparse online time building and updating decks.