Azure Key Vault provides a way to securely store credentials and other secrets, but your code needs to authenticate to Key Vault to retrieve them. Managed identities for Azure resources helps to solve this problem by giving Azure services an automatically managed identity in Azure AD. You can use this identity to authenticate to any service that supports Azure AD authentication, including Key Vault, without having to display credentials in your code.
Reference:
https://docs.microsoft.com/en-us/azure/key-vault/general/tutorial-net-create-vault-azure-web-app