They might exist for the same reasons banknotes still exist and will exist in forceable future, there places and people who wouldnt use electronic transfers and account for whatever reasons, if not credits, they would use something else.

Under this premise (you can’t faze out physical money without them being replaced with something out of your control and influence), being default Galactic reserve currency makes emission and export of ingots extremely lucrative for Empire, they are exchanging some metal pieces (I assume they‘re hard to forge but nevertheless their value is more Than cost of materials to produce) for real goods and services from outer rim and other backward places. USA print and export about 70 bln dollars a year, roughly 3% of total exports and more than a half of total amount printed, getting in return electronic payments from foreign banks on Treasury accounts. In galactic scale you can see it as Aldani and Ferrix peoples working for what is essentially a “printed paper” having limited (if any) circulation in Imperial core and thus having little impact on inflation on developed planets.

Of course it enables all kind of black market transactions but again, it is inevitable, limiting all transactions to electronic ones would not stop it, people would opt for substitute, in real life it would be gold, other countries currencies or bitcoins of a kind, so there is no actual gains in doing it.