Well, it would never be 100% accurate (which is why @hugo has argued against it in the past) as the precise exchange rate you get on settlement can’t be predicted. But the idea is that you have an approximate foreign currency balance displayed alongside your accurate GBP balance to give you an idea of how much you have in local currency. Or maybe you tap on your balance and it shows an approximate local currency balance. Would be a quick way to check what you have in terms of local currency.
However, now we’re well away from the days of a prepaid card, I’m not sure how useful that would be, as knowing my full current account balance in a foreign currency is of limited use. But maybe having the option of seeing a travel pot balance in the local currency. Or tapping on your balance when in a foreign country brings up a quick cheat sheet using the current Mastercard exchange rates (e.g. £1 = €x; £10=€x; £50=€x; £100=€x).