It's a way to get a big fat paycheck. He looks happy about it.
-Paid vacation in Australia
-Opportunity to stay in match shape after the World Cup, as NYCFC doesn't fully exist yet.
-Money, money, money.
Sucks to see him leave all ambition behind at just the age of 32, especially when he was so close to winning another CL last year, but his legacy is more or less set, so why not make a fortune having some fun? At his point he's more of a role player at the La Liga level, and while he seemed content with it last year, he got sick of it at Barca, and this way he can be the main man again.