I'm a bit confused as to why they don't just keep the currency values in the smallest unit for each currency. They gave an example about un-used coins which see

I'm a bit confused as to why they don't just keep the currency values in the smallest unit for each currency. They gave an example about un-used coins which seemed to be their justification, but I can't figure out why adding a "scale" is a better approach. Most of the other examples were backed up by the type system, so I feel like I'm missing something.