To add to the excellent summary of patio11:
The other side of the coin is that more and more exchanges are using Tether (USDT) as the only thing pegged to the FIAT world in any way (value wise), not having to deal with actual dollars and banks makes the legal side of running an exchange a lot easier and cheaper. So a lot of exchanges are supporting USDT for a reason way bigger than simply providing a trading marketplace for them. Think about exchanges like Bittrex and Poloniex, which volume wise are huge and don't offer any real dollars or euros.