When is PrivateKey password used?

[Zmuda, Matthew R]

Looking at the IdPCredentials docs - https://wiki.shibboleth.net/confluence/display/SHIB2/IdPCredentials

PrivateKey - The private key file for the credential; only one PrivateKey element is allowed. The PrivateKey element may include a password attribute with the decryption password for the key.

When would the privatekey password actually be used?

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