During the Empire's heyday, Coruscant was the purely central command center.
It was heavily dependent on Coruscant, which put a huge strain on it.
It was natural that there was an Emperor, and its security and defenses were the strongest in the galaxy. It symbolized the Imperial system at the time.
Therefore, if Coruscant were to be attacked by surprise, it would affect the system.
It would mean that the unified command center would be attacked, so no matter how strong its security and defenses, it would be vulnerable. No matter how strong it was, it would still be vulnerable. All they had to do was attack Coruscant.
These concerns of the Empire gave rise to the idea that Coruscant itself should be mobile. By moving, unlike a centralized system, the risk of being attacked by surprise could be reduced. However, Coruscant is a planet, and it cannot be moved.
Even after the collapse of the Empire, development of Star Destroyers was not stopped, and the First Order continued to purchase advanced technology.
With the advancement of technology, Star Destroyers that are completely independent from the ground have finally been realized! Snoke personally rode on it to command it as a mobile command center.
The cost that the Empire had borne on Coruscant, that is, the cost of maintaining the system, was greatly reduced.
It had almost all of the functions of the command center that Coruscant had, so Snoke had no trouble giving orders. The risks of centralization were overcome!
(It even has the ability to manufacture weapons internally.)
The First Order denies centralization, so a mobile Coruscant is convenient. It is the exact opposite of centralization.
Furthermore, the First Order was prepared for the possibility of Snoke's death.
In other words, even if the Supremacy is destroyed, the system can deal with it flexibly. Therefore, not all of the staff are on board the Supremacy. It is decentralized.
From the New Republic's perspective, they don't know where Snoke is. They don't know the true nature of the First Order. It becomes impossible to launch a surprise attack. Conversely, Snoke himself could launch a surprise attack!
It looks like a dictator aboard a giant nuclear submarine.
It symbolizes the advanced First Order.
*Although the Death Star can move, it is ultimately a planet-destroying weapon and cannot serve as a complete command center.