Fax has been around for a long time. In fact the first patent for a fax machine was in 1843! But what makes fax significant is that faxes are a legal document. That is why when you apply for a mortgage or do most business transactions the faxes start flying.

Emails generally are not considered legal documents for transaction purposes and thus the forever old fax is still with us and may be for a long time. Internet faxes are legal documents. The big advantages of Internet faxes are they are faster, easier to store and are infinitely more secure than traditional faxes on a fax machine. Think about it; with a regular fax machine you need to be out in the open sending secure documents or if you're on the road you need to trust your documents to a hotel clerk to deliver to you! With Internet faxes faxes come directly to your email and no one else sees them.  also see How to send an Internet fax

To summarize:

Faxes and Internet faxes are legal

Email, not so much.