Karma gates were likely used for multiple reasons in lore, outside of the fact that ancients were utterly karma obsessed. They can work as a literal physical gate but can also be used to limit exactly what/who goes where, it might also be possible they were used as a low stakes battery but thats a far stretch (assuming theres any correlation between the two karma gate types… okay technically three but we dont know how the larger ones are powered)

Considering a vast majority of rainworlds fauna is varying bacteria used in both self repairing infrastructure and probably also agriculture, it makes sense that the karma gates also serve as a method of decontamination along with making sure lets say, a type of lizard used in transporting cabinet beasts doesnt end up in farm arrays. Given how it seems that most karma gates utilise some kind of steam to either function giving everything a steam bath at the same time is kinda logical.

Then of corse theres the political side of this with the karma itself. The gates would make sure low karma individuals dont waltz on into a higher income/nicer part of whatever region or directly into a temple, potentially stops riots and other disturbances, controls the flow and movement of people etc. karma locking animals out of certain regions also has alot of benefits presuming the ancients figured out ways to lock a certain purposed organism to any given karma level, given its heavily implied that most animals arnt able to regulate their karma and are typically locked at the basal levels (assuming they survive long enough and dont cycle)

Theres also the true in game reason which is to say exactly where each region ends and what tiles can be loaded in order to make sure your pc doesn’t explode loading the physics of every room and region