Like for example in the world of finance, where you have to be able to backtrack.
Meaning such 'contracts' are least useful for things like, well, 'currency' and 'stores of value' ... hmm ...
Yes, a blockchain gives you an (ideally) immutable foundation. No, that doesn't mean that every transaction that invokes a smart contract has to be immutable. If a smart contract for a particular use case needs to have the ability to "backtrack", so it can, there's nothing stopping it.
Needs backtrack, code it in, then everyone knows how the rules work and there isn't any side deals or exceptions.
Cheers!