Sasuke left to train with Oro and was able to greatly surpass Naruto and be others in that time. With this power he took out Deidara and Itachi for them. His departure also served as motivation for Naruto's training, which no doubt helped in Shippuden. Lastly, with the death of Itachi Sasuke obtained even greater power through his eyes and during the war arc Sasuke joined forces with them, becoming one of the Shinobi Alliances most nessesary assets against the Juubi, Juubito, Madara, and Kaguya. If Sasuke had never left to train he likely wouldn't have obtained the power to go head to head with Akatsuki and Naruto may not have had the motivation to push himself as much as he did in order to match Sasuke. Sasuke would probably get if he ever went up against Deidara or Itachi without that training, and either way he wouldn't be able to help as much as he did in the end game, and Kaguya would have been victorious.

