Minecraft wiki entry for Exploring_an_ancient_city, associated text: An easy way to obtain sculk blocks without a sculk catalyst.
Obtaining soul-related blocks and basalt without going to the Nether.
Easy way to obtain skeleton skulls without having to work with charged creepers.
The useful Swift Sneak enchantment, which is exclusive to ancient cities, as well as many other good enchantments.
(Possible upcoming content) You can obtain Echo Shards from here, which can be used to craft a Recovery Compass.
Has the completely blast-resistant reinforced deepslate, although it cannot be pushed by a piston anymore.
Good way to get treasure like Diamond Armor, Gold, Enchanted Golden Apples, Saddles, and the like.
Mundane items like Candles, Chests, Lecterns, Campfires, Dark Oak wood and Torches can also be looted from here as a nice side-bonus.
The city can be converted into a mob-proof base after all the sculk shriekers are removed, as hostile mobs are not allowed to spawn there.