You can code fax aliases using normal sendmail aliases. At first glance, this fax alias stuff looks like a different way to do the same thing. There is one nice feature here: you put an access rule with the fax alias. You can't do that with sendmail aliases.
Internet service providers will probably set their fax gateway with rules allowing faxing for their operation -- rejecting all other faxing, but including their own customers. In this way, they are providing something nice for customers without good email access, yet they are not providing the world with a fax gateway.