Itachi's death was important because the manga would have been a lot shorter if he was alive. He would have totsuka blitzed Pain before he could destroy Konoha and Obito would have been too afraid to wage war with Itachi around. No war, no villains, Naruto would stagnate.
His death was well timed and saved the avenger
He didn't save sasuke from being consumed by the darkness Itachi entrusted this job to Naruto
But his death was vital from this point sasuke was manipulated or else he either would have come to the village or would have lived a life of vagabond
But from this point he became more like a dark Knight