AWS Secrets Manager gives me everything I want, for no effort, and costs rounds-down-to-zero. I cannot begin to imagine what Vault could even offer that would tempt me to switch.

Say you want to increase security by issuing short lived accounts and passwords in a database, and make these accessible on demand for apps needing them.

The app instance authenticates using the cloud providers key vault and from here it’s allowed to use a policy claiming tokens granting rights to a specific db role. These tokens are short lived and are automatically renewed.

This is just one case - vault have a lot of useful integrations that let you “float above” any cloud:

https://www.vaultproject.io/docs/secrets

I like to keep my concerns separated. That way we can use any cloud as IaaS and keep the platforming part ourselves. Vault is at the heart of achieving something like this.