Unforeseen circumstances like possibly a hospital visit or my girlfriend's Buffalo getting sick!
But I think in reality in a modern world money should move around immediately if it's transferred electronically. The banking sector in Australia just went through a massive overhaul a few years ago to facilitate this.
Personally, I don't know when or if I'll need it, but I'd rather have it than not have it. I don't like to keep a lot of money in Thailand not working for me so I top up as the bank account gets low. Of course there are other ways to manage that, but I'm taking this action because this is my preferred option.