I have used the solution on a couple of projects for a client, mainly for storing credentials and secrets, such as API keys and application or username passwords into the vault, as well as certificates.
It is used for anything we need to keep safe and secure and not have users access, except via applications that programmatically access Key Vault and retrieve the secrets and connect to other APIs. That way, we don't supply usernames and passwords within application code or to people. We vault them in Key Vault and those secrets can be used within an application without human intervention.
Azure Key Vault is a SaaS solution.