The reason Palpatine saved Darth Vader after he suffered critical injuries in Star Wars: Episode III - Revenge of the Sith has something to do with Darth Maul. The Star Wars prequel trilogy is essentially the story of a chess grandmaster, as the Emperor manipulated the entire galaxy in order to achieve his own ruthless goals - specifically, in creating the Galactic Empire.

Palpatine considered everyone else to be just pawns in his game, to be used and then discarded. That was neatly symbolized by the succession of Sith apprentices he ran through; Darth Maul served his purpose on Naboo and was then forgotten, Count Dooku became a convenient scapegoat for the Clone Wars and was sacrificed in order to help seduce Anakin Skywalker to the dark side. But when Darth Vader was crippled on Mustafar, the Emperor sped to his side and exerted surprising effort to keep him alive. What had changed?

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Darth Vader was the Chosen One, with a higher midichlorian count than even Master Yoda. No doubt the Emperor - who desired to conquer death, and had learnt the secret of Essence Transfer from his own master, Darth Plagueis - was intending to inhabit his body with his spirit and claim that raw potential for himself. Darth Vader's defeat scuppered that plan, because Palpatine would never desire to live in such an imperfect and scarred vessel. But the Emperor had still exerted a great deal of effort to acquire Anakin Skywalker as his apprentice, and he was unwilling to simply discard him. Part of that has to do with Darth Maul's survival on Naboo.

Darth Vader in the Revenge of the Sith

Star Wars: The Clone Wars shines an interesting light on Palpatine's decision. According to Darth Maul voice actor Sam Witwer, Maul had impressed Palpatine by returning when he was believed dead, and becoming a key player in the galactic conflict. As he explained in an interview with Hollywood.com back in 2013:

"What’s really fun to note is that Darth Maul, having survived his grievous injuries, plants the seed in Palpatine’s head that maybe you don’t give up on these guys so quickly. If they’re dead, or you think they’ve been killed, maybe they’re not. Maybe they can survive, maybe they can amount to something. That’s something we get to see coming up, Sidious realizing that. That ultimately leads to Darth Vader."

Maul had survived being cut in half by Obi-Wan Kenobi in Star Wars: Episode I - The Phantom Menace, and had returned to become a power player. Palpatine knew Darth Vader had far more potential than Maul, so decided not to write him off. Instead, he had some of the galaxy's best cybernetic experts figure out how to keep him alive, as detailed in Charles Soule's first Darth Vader run. Once that was done, though, he put Vader to the test in order to see whether he was still worthy to be his apprentice. These tests are detailed in Charles Soule's second Darth Vader series, and in Paul S. Kemp's Lords of the Sith. Much to the Emperor's delight, Darth Vader decided not to allow his injuries to limit him, and instead focused so deeply on the Force he became more powerful than ever before. He would still have unsuitable as a host for Palpatine's spirit, but he was nonetheless a perfect tool, even in his crippled state.

More: Star Wars: The Rise of Skywalker Explains A Darth Vader Mystery