Exchange rates can be extremely volatile. Say (for the sake of the argument) you live in the USA (USD base currency), went on a trip to Europe half a year ago, exchanged money from your main currency (USD) to the local currency (EUR) at the exchange rate back then (1 EUR = ±1.25 USD). However, after the trip you had 300 EUR left in your account and left it there. Say you are coming back to Europe in the near future, but the exchange rate is much more favourable for Americans now than it was back then (1 EUR = ±1.15 USD).
When you’d pay 100 EUR for e.g. dinner with your spouse on this next trip, it would be deducted from the 300 EUR you have left from the previous trip as the account in the local currency has sufficient money in it. This means you’d be paying 125 USD (100 EUR * 1.25 USD/EUR back when you exchanged this money).
Now my idea: if you could chose to ignore this local wallet whenever the current exchange rate, and thus conversion on the fly, you’d only be paying 115 USD (100 EUR * 1.15 USD/EUR current exchange rate). This is way more favorable and you’d be able to save quite some money that otherwise would just be lost.
Now, I did found 2 workarounds suggested by other users in this forum (it was suggested for something else but does apply to this situation as well):
A. put the currency you want to save in a vault,
B. deactivate a wallet by going to More>Profile>Account details>swipe left on the account you wish to deactivate>Deactivate.
Note: All examples, currencies, exchange rates, etc. are just random, easy values for the sake of argument.
Personal note: Yes, you could argue that you’re only saving about 8%, which in this example would be about 24 EUR (30 USD in the new exchange rate). However, for a student like myself this a night of going out to a local pub with friends.