This world is believed to be an extrasolar capture that built up its atmosphere over millions of years to become the hothouse it is today. Volus robo-miners descended on Poloh Tem when element zero was first discovered there, and in turn, pirates descended on the volus. The pirates then found out, to their detriment, that wealthy volus can buy a lot of influence in the Turian Hierarchy.
The Reapers occupied Poloh Tem's orbital stations rather than destroy them outright. It is possible that element zero is rare enough that the Reapers harvest it--but it is more likely that the Reapers are stationing ships here in case the turian forces they drove off seek to recapture it.