The point of stablecoins is to have something that's the same value as a dollar, but which can interface with ("all the wonderful magic of") smartcontracts.
One way to achieve that is to have a big trusted organization honor redemption on demand. (GUSD and, I think but don't quote me, USDC.)
Another way is for an org to consistently honor enough redemptions (but not on an on-demand basis) that it trades at par (USDT, for now).
Another (dubious, increasingly unreliable) way is for some cryptocurrency to automatically buy the stablecoin when it falls below par (TerraUSD/UST with LUNA, already failed).[1]
Yet another is to issue the stablecoin only in return for a significant collateral buffer (with other cryptocurrencies) and force sellbacks when the collateral gets too close to critical (DAI and, I think, FEI, not to be confused with the DEI in this story).
[1] TerraUSD's platform had the Ponzi-esque Anchor paying 20% returns, but is stabilizing mechanism did not (attempt to) depend on this in any direct sense.