Many travellers will still make the effort to get currency, presumably in a bid to avoid fees for using their cards abroad
...and they are short-sighted. They believe (wrongly, just about always) that the exchange rate they get is the same as the rate used for card purchases. Typically, it's 3 - 5% worse. Even if it's claimed to be "commission free". A
good credit card will use something very close to the city "middle" rate and won't charge anything extra. Some few even give a small % cashback on top.
Thus, if I use one of my cards vs. taking cash and spend USD1000 (online rates just now):
Card cost ~£775 minus £2 cashback = £773.
Cash from the post office £791.95
Cash wastes almost £20.
Ignores market fluctuations that may occur between the time you might have bought currency and the day on which you actually spend it. Those are wholly unpredictable and might work equally for you or against you. The only thing that any form of pre-purchase (whether cash, transfer to a currecy card/accoiunt/etc or other) gives you is a fixed (likely worse, at the time) rate at a point in time.