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A concrete example would be flash loans, where you could borrow some value from a lending script, do something interesting with it, and return the borrowed amount to the script with a fee, all in one transaction.
Intuitively this feels like it would need to be multiple transactions with separate outputs, but the lending script needs to verify that it is receiving sufficient value in return before it will ever lend anything.
How might this be modeled in Plutus to work?
Transactions on Cardano can be made up of many state changes. The transaction as a whole is atomic and a state change in the transaction will only be performed if all the changes can be performed.
For something like flash loans, as @Marek-Mahut points out, instead of having multiple transactions imperatively executed, each transaction just includes the before and after state. Flash loans are no longer needed in Cardano's model.
You could have a contract that allows you to redeem some ADA as long as you replace it with some interest in the same transaction--which is the equivalent of a flash-loan--but why would you ever do that? Plutus validators don't check that they're getting paid, they check that the state after redemption will be valid. The flash loan no longer serve the purpose of unlocking liquidity inside contracts. That comes for free with the EUTxO model.
tl;dr: All transactions on Cardano are made up of atomic state changes. This allows for what would be atomic sets of transactions on other blockchains.
Flash loans are not possible within the eUTXO model as each transaction is a single state transition, where a typical flash loan consist of two state transitions within a single transaction.
In my opinion, this is one of the most interesting security/safety features of the eUTXO model.
