Minecraft wiki entry for Best_biomes_for_homes, associated text: Immediate access to the underground's vast mining resources.
Plenty of stone, dirt, ores, etc.
A cave, especially one that is big and/or opens up at the surface, can be transformed into an improvised home.
The player may find structures like strongholds or mineshafts suitable to easily change into a home.
Due to their trades, wandering traders that spawn underground can serve as underground shops for useful resources that would otherwise require the player to go to the surface.
Perfect for a secret base/room.