For states:
They quietly inflate the money supply by forking fiat, achieving monetary base expansion without the political cost of explicit money creation
For issuers:
They convert user deposits into a private mint: risk-free interest on collateralized reserves, with none of the upside shared with holders
For users:
For everyone but the unbanked & criminals, stablecoins are strictly inferior money surrogate: no yield, no guarantees, and no recourse