Barcelona getting him just because they can, they don't even need him in the slightest. Busquets, Rakitic, Vidal, Coutinho and co. aren't enough?
When will Arthur and Aleña even play?
If I'm not mistaken they wanted Rabiot this summer and sent Abidal to make a deal, while they expected to only get Arthur this winter. So it makes sense if they simply switched when they realized that PSG wasn't negotiating.