Because he'll really get playing time there, versus Ribery, Robben, Shaqiri, Gotze, Muller, Mandzukic, probably Lewandowski, Pizarro and Kroos? + any other players they bring in/through this summer...
And I don't see why a big club would want to bring through a Barca player, who will definitely...
Barca don't need money though, the primary aspect here is the player's development. Is it best for him to be in a top team, training with top players but maybe not starting, playing regularly or playing off the bench - or to be in a lesser team, still a physical league, but playing every game...