Bar codes on cash couldn't track all spending unless it links to who the purchaser is. If I walk into a shop and give a retailer $10, the government just knows someone gave that vendor a genuine $10 bill. The retailer can give me change in cash without scanning it (because they already know what they have is authentic), and now a random person has untracked bills they can give another retailer anonymously. That means they can't really track much of what's in circulation—unlike they can with credit/debit cards.

If this came with an elimination of income tax (the government excuse to track spending), there'd be no excuse to demand bills be scanned except to check authenticity.

And it would make sure bad actors could not inject fake cash into the system and, I suspect, would make it harder to launder money safely.