I still see no use-case.

The funds in my bank account are already digital and, when I make a purchase with my debit or credit card, Google/Apple-Pay (or whatever other service), PayPal, etc. the process takes place entirely in the digital realm.

It's all traced and taxed already.

It's all recorded already.

In what way would a 'digital currency' be different?

It wouldn't - it's simply reinventing the wheel.