Currency exchange. In the banking system it’s hard to exchange currencies for a fee lowe than ~0.5%, even for amounts in the tens of thousands USD.
If adding another layer was somehow more efficient, then all the companies in this space would do it under the hood. Yet almost none of them do.
The reason why DeFi exchanges offer low fees is that they are not “companies”, they are fully automated and have no overhead. For a company offering forex services the main expense are salaries, not fees (that on wholesale markets are on the order of 0.01%), therefore they would not benefit from using a blockchain as an underlying layer.