There were multiple story details that Star Wars: The Rise of Skywalker didn’t make clear, one of which was exactly why even though Palpatine had survived his apparent death at the climax of Return of the Jedi, he waited until after Supreme Leader Snoke died to make his grand retrurn.
It’s been revealed that Snoke was a clone, one of numerous copies kept on hand presumably as spares should something unexpected befall one of them while he was still useful. Palpatine used him as a puppet to facilitate his control over the First Order and the corruption of Kylo Ren, while keeping his own survival a secret as he recovered from his resurrection. And it was the latter of those two uses that was far more important to the former emperor.
Although neither Snoke nor Kylo were Sith in the traditional sense, they still operated by the Rule of Two established a thousand years previously, that the Dark Side power structure should consist only of a master and an apprentice, one to wield power and one to crave it. Through Snoke, Palpatine oversaw the training and seduction of Kylo to the Dark Side, and when Kylo killed Snoke in The Last Jedi, it demonstrated to Palpatine that the lost Jedi scion was ready to assume his place in the Final Order, in much the same way Palpatine killed his own master Darth Plagueis many years previously and took on his own apprentice Darth Maul in the first move of his plan to bring about the destruction of the Jedi.
Not explaining this was one of the many narrative oversights of Star Wars: The Rise of Skywalker that its novelization clarified, explicitly stating that Snoke’s presence was a test, with Palpatine congratulating Kylo for killing him and proving himself worthy of being the Sith Lord’s successor. It’s just a shame that the movie didn’t bother explaining this.
