from here:

As someone who’s spent a lot of time in Scotland, this idea appeals to me for a non-practical reason, too: I like the variety of banknotes one finds when the notes are issued by a range of different banks. (For historical reasons, Scottish banknotes are actually issued by private banks: the Bank of Scotland, the Royal Bank of Scotland, and Clydesdalel Bank. The same is true in Northern Ireland, of Bank of Ireland, First Trust Bank, Northern Bank, and Ulster Bank.) This can make inter-national travel rather interesting at times, but it certainly makes the world a more colorful and heterogeneous place.