The earlier posted rules of the format, the rules as of yesterday, allowed you to steal up to 165 points of your opponents' army, or one card. So if I brought Stingers x5, Charos, Marcu, in the old rules you could steal Charos (since he's one card) or Stingers x2 and Marcu for 140 points. In the new rules, you can take all 5 squads of Stingers, or Charos, or Marcu.
The effect of the old rules was it massively encouraged you to play figures that messed with that 165 point cutoff. The Minion x4 Atlaga build I posted earlier was the most egregious one I could think of; your opponent can only steal 110 or 90 points from you, since Atlaga and Minions don't add up close to 165. And what are you really going to do with one squad of Minions or Atlaga against a bunch of squad figures. The Mezzos x8 build that Orc posted only lets your opponent take 130 points, and you can steal any special attacker build they might bring. Basically the meta would have been huge piles of high point squads like Minions Mezzos Hounds duking it out.
The effect of the new rules (the rules of the format as it was designed by kevindola back in the day) is that neither one of those armies are close to viable. If you bring 8x Mezzos your opponent steals your whole army and you instantly lose. You are forced to bring an army with many pieces, none of which are essential to the army's success, since if it's essential your opponent can just take it.
I don't know what advantages the old rules offered, to be perfectly honest. Normally I wouldn't be a fan of changing the format this late but in this case the change helps the event be so much more fun that I am on board with it.