UST staking had nothing to do with PoS though. The Terra blockchain's native cryptocurrency was LUNA. There was a relationship to LUNA, in that $1 worth of luna burnt would mint $0.999 UST, and vice versa (1 UST burnt would mint $0.999 of LUNA).

UST was appealing because you could supply it in Anchor protocol, which was a money market. Anchor advertised nearly 20% APY on your supply, so it was appealing to convert your fiat to UST and then supply it to Anchor like a high interest savings account.