I still don't understand what is the advantage of having a stablecoin...if I need to have $ or Eur I think it's better to change crypto to FIAT. Also there is no stablecoin that is guaranteed by a trusted third party escrow.
Not all exchanges offer fiat trading pairs, so stablecoins are quite handy to trade on decentralized exchanges for example.
Now, whether it's safe to hold your funds in those stablecoins is a whole different story...