smart contracts existed way before blockchains.. but they did not work, because there was no audit mechanism nothing to peg it to to ensure duplication can occur.
it was the main bane in cypherpunks lives, trying to solve how to make smart contracts actually not become multiplied/double spent/counterfeited.
satoshi invented the blockchain as the invention. and bitcoin as the utility example that shows its function.
many silly people are trying to invent new networks that dont need a blockchain. yet they then stumble back into the flaws of what the cypherpunks were falling into 13+ years ago.
the blockchain is:
a ledger, log, registry
but also part of the invention is the rules about the blockchain. like it being:
an auditor
an immutable way to prevent double spending/counterfeiting
requiring expense/work to collate the data,
incentivise said work with reward
ensuring the work is not easy to just be abused
not cheap to manipulate but more rewarding to not manipulate
if it was a centralised blockchain(one copy on one server). then thats a central point of failure.
failures of power, hardware glitches, sabotage, internet outages, service outages in bad weather, etc etc
which is why a decentralised blockchain has its advantages
having a entity of employee's using one reference client but distributed across many different locations. can preserve some security against a single employee 'pulling the plug' or editing the ledger. and the remote locations can mitigate against weather, internet, electric and single serve r failure.. but again if they are all relying on the same software, they all have a problem if there is a software bug.
having a single dev team working on one reference client is a central point of failure, because if their is a bug in that brands software everyone relies on for the protocol, everyone has a problem.
diversity of random people not connected or in relationship to each other all maintaining the ledger is the other key piece that satoshi solved with bitcoins blockchain invention
and back then there WAS also a diverse base of different 'reference clients' all sticking to the main rules but using different implementations on how they do extra features beyond the basic consensus rules
(local store in different DB sets)(some had extra rules to double check transactions and blocks.)
this diversity has waned somewhat in the last 7 years.
a CBDC has advantages over normal digital fiat.
EG having blockchain nodes per bank branch means there is no single point of failure of hardware risk(pulling plug, electric/internet loss, weather hazards)
EG no single employee can defraud the system
but.. here is the but.
it all depends on how they allow bank customer/citizen access to it
if its a multisig setup where a bank branch has a key of a 2of2 multisig. then banks can still refuse to sign.
if its a multisig setup where a bank branch has a key and bank HQ/IRS has a key of a 2of3 multisig. then bank branch + HQ/IRS can seize funds without citizens consent/authorisation
if banks are incharge of the key creation they can veto out utility of certain people not even getting a key. EG undocumented immigrants with no ID