low income investor is spot on...
It's the ultimate nudge system. As I understand it, with a digital £ the government can not only track exactly what is being spent, but also control the money in our bank accounts in whatever way it deems appropriate, for example, by applying negative interest to any money not spent, or deterring / forcing certain kinds of expenditure, since there is no way to withdraw it as cash. Digital cash could also have a limited life, since it wouldn't be possible to turn up to the Bank of England demanding payment of the amount promised (although I suppose that hasn't been realistically possible for some time). That's not to say that governments would definitely act in such a way, but they would have the power to do so.
I'm not aware of any particular benefits to the citizenry, since we can already spend and save electronically if we want to. I suppose the major positive would be the inability to commit fraud by avoiding charging or paying VAT, but I'd hope there might be better ways to achieve the same goal.
NB government digital currencies are not the same as cryptocurrencies. Those in favour of the former are probably unlikely to support the latter and vice versa (although I have not done any research on this).