What are the reasons to underpot?

[Carlos]

Hi everyone,
just like the title says, why? as far as I can uderstand from browsing these forums is mostly because if you overpot you will most likely have root rot as the medium will take longer to dry out inbetween watering especially in the middle of the pot. Those that are experts with orchids wont have a problem with this (believe me, this is not my case ). Some say because some orchids like to have the roots tightly but having looked at most orchids habitat, i am mostly thinking Phal. their roots are spread across the trunk of the trees and therefore very spread out. Is there something i dont know that the expert know?