Just as here on HN we read from time to time about people being locked out of various online services (i.e. Google, Facebook etc.) for doing something a company doesn't like, a CBDC would allow governments the ability to effectively lock people/organizations out of their digital economy.[1] It would also allow for money that expires and can only be used for certain (government approved) purposes. (these both would have been desirable from the government's point of view for COVID stimulus payments, for example)
[1] This can already be done to a degree via the banking system, but a CBDC would provide for much more granular control.