I think it's a dream for most players of his generation, incl Real Madrid. Ronaldo left an English giant at their peak to go to Spain. Plus, it's not like Barca is completely irrelevant, they did just win the league title.
Sometimes players also relish the opportunity to be part of a rising project. There's the allure of hopefully becoming an important piece in a team that's trying to establish or re-establish a foothold among the European top dogs. That's how we managed to get the likes of Yaya Toure and David Silva, and how Newcastle managed to nab Bruno Guimaires.
Sometimes players want to be the 'Yaya Toure of Manchester City' rather than the 'Yaya Toure of Barcelona', if you understand what I mean.