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If you didn't want to "go public" and reveal you were still alive, then you'd want an alternate.

Ultimately, I don't think anything about this plot was thought out (or thought about at all, really), but if I'm taking a stab...

My best guess is that the Emperor wanted to wait to announce his return. Why? I don't know. It seems like announcing, "Hey, the Rebels didn't kill me! I'm immortal!" would boost the morale of troops, terrify foes, and prevent any power vacuums from occurring. But what do I know? I'm not the god-monarch of a galaxy. Possible theory: Snoke was a real person who claimed power after the Emperor's defeat on the second Death Star and the Emperor needed time to reform his consciousness in a new body...?

The vats filled with Snokes then are the Emperor going through "Beta Versions" of the Snokes. Whether he's cloning a real dude or building him from scratch, he likely needed to get it right. Maybe the Kamino cloners weren't available.

He then replaced real Snoke at some point in time with a clone Snoke and killed the original. The Emperor then waited until he thought it was time to announce his return for dramatic effect.

Deformed gimps wearing what can be best described as golden bathrobes. Honestly, Rian Johnson was taking the complete and utter piss when he set the wardrobe for Snoke.

Snokes in a Jar. The ghost of Mace Windu was heard to remark: "I've had it with these motherf-ing Snokes in these motherf-ing jars!

It's what happens when you force JJ "Mystery Box" Abrams into actually delivering on any of his nonsense. His creative abilities seems limited to things like "hey, how cool if there's this giant hologram of a messed up looking dude?" knowing that someone else will have to eventually explain it. He invents goofy mysterious stuff while planning to fake out with more until other people come along to finish it. Making him do Episode IX probably caught him off guard and this was the best he could do.

He wanted to keep his identity secret. He comes right back as Palpatine, BAM. New Rebel Alliance/Resistence. Snoke allowed him to take his time, plan, build things up, etc. His new Empire fleet took decades to build.

Plus, it seemed like the Snoke body didn't deteriorate like his new one. He was barely alive. Machines and the Force kept that decaying body barely afloat. Palpatine could possess/control it from a distance.

I'm more puzzled about how the massive fleet of Star destroyers worked.
Thats quite a lot of hardware , and where did the crews come from .
He must have had to have had about 20,000 people on a retainer come and crew these at a moments notice .
...or they were in stasis on board or whtever ..