There are many good reasons to use MyFax - it's 75% cheaper than regular fax machines, lets you get your faxes anywhere, simplifies your office, etc.
With increasing scrutiny on corporations - and the regulations to go with it - comes another reason for using MyFax: compliance. Sarbaanes-Oxley, HIPPA, PIPEDA etc. all drive a need to control and track the flow of information, and fax machines can be a gaping hole.
Here's a recent article on the topic, at Compliance Pipeline: Faxes: The Achilles Heel in your Compliance Armor.
Here's more information on how MyFax helps with regulatory compliance and privacy.