Largely to ween ourselves off the dependence of Visa and Mastercard, who are currently skimming off huge amounts of money across the whole world by having a near duopoly on payments. Cryptobros are wrong about a lot of things, and proof of work / proof of stake are both terrible in their own ways, but they aren't wrong about the problems with the traditional payment systems (even if bitcoin et al introduce more and worse flaws).

A digital currency backed by a central bank gets rid of the wasteful mining (the BoE is allowed to just create currency from nothing, that's its job) and provides the regulation that is so badly missing from crypto—which is why it is so scam infested.

1. It becomes impossible to avoid taxes.
2. The flow of money through the economy would be easier to trace and examine.
3. Increasing and decreasing the monetary supply becomes trivial to control inflationary or deflationary pressures.
4. money laundering becomes difficult, since every transaction can be scrutinized.

The only useful purpose for crypto is as fiat currency. Whether that’s a good idea is another matter, since it would eliminate transaction privacy.

Since governments are becoming more fascistic, I generally think this is a bad idea, since it would give governments more power (to do good or evil).