Idea is, if someone gets access to your CI environment but not the repo - they don't have the data to decrypt. E.g. if your CircleCI password is leaked, but GitHub is safe. In my experience, most CI websites don't allow to browse the source code.

That's not ideal - IIRC, CircleCI in particular has a feature to run the build with SSH, so a sufficiently aware attacker still has a way to get the decrypted data. Still, an extra obstacle. Just that security's not binary.

Also, this way you can manage the secrets in your repo. No need to log in to CI and use their web UI to edit the passwords. I think that's convenient.

Sops is a better idea, because you can't extract keys from the KMS.