I think 20% is just an arbitrary number the witnesses selected. Each has their own thinking for the rate they set. The real question is, why pay interest at all?

Suppose the market tops with $1.50 HIVE. You’d want to lock in your gains. Without HBD, you’d have to sell HIVE on the market, pushing down the price.

With HBD at 0%, it’s an easy conversion without having to go to an exchange. But if your gains can get 5% at an exchange for depositing your stablecoin, you might cash out HIVE or HBD to park your gains there.

Interest entices the HIVE community to park gains on-chain. We were quite happy at 10% at outset. But suddenly they bumped up to 20% without explanation. Someday the witness consensus will lower it without explanation.

For now, the community is voting on 20% witnesses. That’s really all there is to it. Random people voting.