The latest Star Wars tie-in, Timothy Zahn's Thrawn Ascendancy: Chaos Rising, subtly explains why Palpatine chose to base his fleet on the Sith redoubt of Exegol in Star Wars: The Rise of Skywalker. The Emperor's shock return from the dead drove the entire narrative of Star Wars: The Rise of Skywalker, but the film itself left a lot of questions unanswered. Even the precise mechanics of his resurrection have only been explained by tie-ins and novelizations.
One aspect of the Emperor's plan seemed particularly bizarre. Palpatine had been resurrected on the ancient Sith redoubt of Exegol, a unique vergence in the Force where the veil between life and death is particularly thin. He had chosen to make Exegol his base of operations, charging the Sith cultists to construct a vast fleet of Xyston-class Star Destroyers there. It was an odd decision, because Exegol's atmosphere was so unstable it was difficult for the fleet to exit. If Palpatine's fleet had been amassed on a slightly less storm-ravaged world, they would never have been defeated by the Resistance and the so-called Galaxy Fleet. So why did the Emperor make such an apparent strategic error?
An answer is subtly provided by Thrawn Ascendancy: Chaos Rising. This is the first book set entirely within the Unknown Regions, and it reveals a lot about the instabilities that make the Unknown Regions so difficult to explore. The species who live inside the Unknown Region refer to it as the Chaos, and it was apparently formed millennia ago by a series of chained supernova explosions. These caused a calamity of galactic proportions, as debris and heavy electromagnetic flux resulted in constantly changing hyperlanes that make the Unknown Regions almost impossible to navigate without the Force. The Unknown Regions' inhabitants, such as the Chiss, have identified a number of what they call "box systems," entire star systems where there is literally only one way in or out. In strategic terms, these are ideal for military purposes; for establishing secure refueling depots, secret rendezvous points, or - most significantly of all - building an army without anybody being aware of your activities.
Exegol is the ultimate box system, because all evidence suggests it can only be found by use of the Force - either by possession of a Sith Wayfinder or through an intense, exhausting Force power called Skywalking. That means Palpatine could safely bide his time there, waiting patiently until his forces were ready to launch into space. When he finally struck, it would be without warning, with no chance that anyone - be they a member of the Resistance, the New Republic, or even inhabitants from the Unknown Regions - was aware of the vast armada he had amassed. Meanwhile, the Emperor undoubtedly used the dark side vergence that is Exegol to amplify his own powers, allowing him to influence galactic events from a distance. He was able to establish a Force bond with Ben Solo while he was in his mother's womb, to manipulate the boy as he grew, and ultimately to destroy Luke Skywalker's Jedi Temple at the right moment.
It's worth remembering that, while the various species from the Unknown Regions were aware of the tactical benefits of box systems, the Emperor's actual enemies were not. The Resistance were far more familiar with the stable hyperlanes of the galactic core, and there's no evidence any of their strategists had ever really considered how to exploit the instabilities of the Unknown Regions. Thus the Emperor's strategy was one that took everyone unfamiliar with the Unknown Regions by surprise, and no doubt he would have triumphed in Star Wars: The Rise of Skywalker if not for the light side of the Force.