He's long held a desire to go to Madrid. As others have said he'll be the main man in the team and the league.
I think going for him now is a statement of intent from Madrid to the fans that they can still compete for signings even if it's more performative than real and they pick him up free next year.
I think he wants to test himself around a couple of leagues and teams and a new deal at PSG will likely trap him there (albeit insanely well compensated ).