It's your primary progression system, and the acquisition of cards is done via loot boxes... and yes, you can buy loot boxes with real world money. (bytesizeimpressions.com)
And will any class - specific items and gear still be found in bought loot boxes? (eurogamer.net)
There you can still effectively buy loot boxes, but you're buying in - game currency with the «gift» of a loot box. (thesixthaxis.com)