Star Wars has finally revealed why Palpatine created Snoke. The Emperor returned in Star Wars: The Rise of Skywalker, revealing he had really been the main villain behind the sequel trilogy all along. Naturally, this required a significant number of retcons - one of which was the unexpected revelation Supreme Leader Snoke was actually a construct, a genetically engineered being through whom Palpatine continued to act in the galaxy while remaining hidden away on the Sith redoubt of Exegol.
Lucasfilm has recently published The Star Wars Book, with contributions by Pablo Hidalgo, Cole Horton, and Dan Zehr, and it attempts to smooth out Supreme Leader Snoke's backstory. It reveals Snoke was more than just a conduit for Sidious' spirit, but he may well have had free will of his own right; presumably a false history implanted within his memories. This may help explain why Snoke's backstory is now so problematic, because he had no idea what he really was. But, of course, this raises an interesting question; why did Palpatine need a proxy in the first place?
Fortunately, The Star Wars Book explains this. The key is to remember that the planet Exegol is a vergence in the Force, a place where "the veil between life and death [is] thin." That is why the Sith Eternal cultists were able to perform a ceremony on Exegol that summoned Palpatine's spirit, allowing the Emperor to possess a clone body. But it seems there was a catch; according to The Star Wars Book, he then became trapped.
"The processes are far from perfected, and Darth Sidious' cloned body rapidly deteriorates, trapping [Palpatine] on Exegol as his frail form is unable to leave. In the confines of his recuperative sanctuary, he plots the next phase of ultimate Sith vengeance, and the final defeat of the Jedi."
This perfectly explains some of the apparent inconsistencies in Palpatine's plans after his resurrection. He initiated Operation Cinder, sending out Sentinel Droids to convey his orders to Imperial loyalists and trigger unprecedented acts of genocide as they ravaged the galaxy at his command. This appears to have achieved multiple purposes; it was a bitter act of vengeance exacted upon the galaxy, a scorched-earth strategy intended to destroy all Palpatine's hidden bases so there were no clues to his resurrection, and a way of forcing out any Imperials who had a conscience. The Emperor intended Operation Cinder to culminate in a battle over the skies of Jakku, a planet he had rigged as a gigantic bomb. The plan was for the galaxy's military to be crippled, while the Emperor's most loyal forces were guided into the Unknown Regions, where presumably they would have been met by Palpatine himself.
But things did not work out as Palpatine had foreseen. As detailed in Chuck Wendig's novel Aftermath: Empire's End, New Republic forces dismantled the planetary explosive at the heart of Jakku. Imperials who headed out to the Unknown Regions discovered a vast power-base had been prepared for them, mostly using droid factories to construct weapons, but a vacuum had been left at the very top - left empty because Palpatine had been unable to leave Exegol. Thus the Emperor created Supreme Leader Snoke as his proxy, to fill that gap, although it probably took time to create him - explaining some of the confusion in tie-in novels.
Meanwhile, Palpatine sought a new host for his spirit, someone he could possess using the ancient Sith technique of Essence Transfer. It's reasonable to assume he initially fixed his eyes were immediately drawn to Ben Solo, the grandson of the Chosen One, potentially heir to all Anakin's power. Again, according to Aftermath: Empire's End Leia sensed a dark, malevolent force watching over the child in her womb, and it is likely Palpatine formed a Force bond with Ben even at this early a stage. But he ultimately learned of his "granddaughter" Rey, and concluded she would be a far more effective vessel. Hence he announced his return to the galaxy, intending to draw Rey to him so he could possess her, and then emerge from Exegol with a fleet of Xyston-class Star Destroyers. Fortunately, yet again, the Star Wars story demonstrated the Emperor's visions of the future could be foiled.