You’ll Use One Account to Play Minecraft, Cobalt and Scrolls
The idea is that you use this Mojang account (where the username would be your e-mail address) to sign in to Minecraft, to Scrolls and to Cobalt. After that you would proceed to choosing which profile you’d like to play as—for Minecraft this would correspond to today’s accounts (premium status, display name, skin etc).
They say you won’t be forced into this if you have an existing Minecraft account — you’ll be given the option to opt in — but I assume you will have to use it when you pick up Cobalt and Scrolls.
In terms of keeping everything tidy, this is probably a good thing. Now, if only Mojang could figure out their issues with Steam so we could just put these games there…