so beside being cool is there any benefit to floating cities Mobility. When the core of your arcane engines, ritual sites, and populace can be shifted towards bountiful sources of power, or away from dangers… it can be pretty useful!
— Matthew Mercer (@matthewmercer) May 27, 2022
Defense: the barbarian hordes aren’t going to swarm the gates. Flying established a bar of entry that eliminates less able/less sophisticated enemies from having any hope. Kind of like an island city rules out attacks from people who don’t have boats and ships. And even if you do have the ability to fly for an assault, how are you going to lay siege to such a city? or even blockade a city where the city itself might evade the blockade? Assaults are the end of an attack on a city, not the start… to get there you have to wear down the defenders by using siege and blockade for attrition… you can’t do that to a flying city. Which means you only have the assault, against a force that you couldn’t wear down via attrition.
Offense: armies are vulnerable on the march. Armies in floating cities are protected while traveling, while resisting the besieged attempts to break the siege, etc. They can also attack other armies/navies that are on the move (via land or sea) without making themselves similarly vulnerable… and can take area control that a fortress gives without having to construct a local fortress.
Weather: no earthquakes. No having to endure harsh winters (move your city from fertile spring/summer lands to warmer climates for the winter… come back in the spring for the agricultural seasons). Same with the seasonal floods that make the farm land fertile, extended droughts, fires, etc. Local conditions are no longer permanent impacts on your city. (some of that is covered by the mobility answer, but the floods and earthquakes benefits don’t have to be about mobility).