#3 Enhanced experience and exposure
Harry Kane has accomplished much and done aplenty at Tottenham, despite only being 24 years old. In spite of this, I would argue that he would be instantly exposed to a higher and greater standard of football if he were to swap clubs with Madrid.
Kane would have the regular opportunity to feature and possibly win the UEFA Champions League and the league, whereas Tottenham (with respect) seem unable to do so momentarily.
Furthermore, playing for Madrid would expose Kane to new competitions such as the Copa del Rey and possibly, even the Supercopa de España and the Club World Cup. In addition to this, he would get to experience the indescribable phenomenon that is El Clasico.
Coupled with the above, Kane would also get to ply his trade alongside some of the best players in the footballing world; the likes of Cristiano Ronaldo, Gareth Bale, Sergio Ramos and Marcelo would become his teammates, and nobody can argue with the fact that playing alongside top-notch talent would surely serve to escalate one's drive to improve and work harder at their own game.
In short, with all that invaluable exposure and experience, there can hardly be any doubt that Kane would only mould into a better version of his current self.