I think the most useful thing about faxes, security-wise, is that in their basic form they require zero digital storage of the image being sent. The only record on either side of the transmission is a piece of paper.*

Contrast that with email, which is store-and-forward by design, and now you have to put in effort to ensure both the sending and receiving email providers delete the message in a timely manner.

* obviously you can add store-and-forward behavior to either fax machine, but it's not the default.