I’m not talking about bonds, but fractional reserve banking, where a bank will credit two accounts with $1 when it has only $1 in cash.

Edit: As it stands now, there is a "lender of last resort" that can create new money as needed to fill in the gaps if those two account holders want to get their money out, so there's no need to distinguish the original vs bank-created money. But you can't create new Bitcoins, so there is a reason to distinguish Bitcoins on the blockchain vs those IOUs.