Passphrase-file paramater in mender client

Hello,

I was trying to look for documenation on mender client.
i.e mender --passphrase-file passkey.file
I know it has a options --passphrase-file

–passphrase-file value Passphrase file for decrypting an encrypted private key. ‘-’ loads passphrase from stdin.

What is the private key used for?

Thanks
Ed