Thanks to BBC Radio 4 this morning...
I now understand why governments and banks are so keen on digital currency. When interest rates go negative, sensible people will keep their money as cash under the mattress/<name your favourite hiding place>. If money becomes entirely digital, alternatives such as gold under the mattress will be used but don't have a fixed relationship with money.
iPlayer: BBC Radio 4 Thursday the 21st of July: "How Low Can Rates Go?"