Trading. Quick arbitrage, day trading, pump and dumps, leverage. The same thing you can do by having dollars on your stock trading account you usually need coins on your crypto account and it would be crazy to have the volatile ones.
Not that unreasonable. The total number of crypto traders globally is estimated at between 300M [1][2] (note: close to the population of the U.S.) $150B is an average of $500/user held as stablecoins, which doesn't seem that far off.
Traders and tax evaders. With stablecoins you can trade on decentralised smart contracts, and they even allow something called flash loans where you borrow the stablecoin, trade it for profit and then pay it back in one rollbackable transaction! Also there are many high yield investment schemes based on holding stablecoins.
