Stablecoins can be useful when it comes to huge cross-border transactions. Most financial transactions are small and therefore cost nothing but large movements in money are inefficient and cost alot. This is one potential usage of stablecoins.
nbody ever uses stable coins for something like that because the costs of transferring money that way is going to be so much bigger than doing it through regular methods because of the fees and also there are a lot of risks involved with this method.
what you may be referring to is "transferring money between exchanges". for example for arbitrage traders who want to buy bitcoin on another exchange with fiat but transferring fiat from exchange 1 to 2 costs a lot.