He did it for a few reasons:-
2 year contract instead of the 1 we allegedly offered
Wanted a different challenge and culture, as did his Mrs - some might even say that Barcelona is a nicer city than Manchester
He just played for one of the best countries in the Euros
He was in the La Liga team of the season
We don’t need him to play every game, 90 minutes - but you don’t think he could change a game?
He is an incredible character, apparently well like by everyone in the squad and on the staff
No transfer fee
He adds to a thin squad, even more strained by the injury to Bobb
He can play defensive mid, attacking mid, false 9
It’s a bit of a what did the Romans do for us scenario for me