Using the Mac as a fax machine is a terrific idea, for a lot of reasons. It saves money on paper and fax cartridges, and spares you the expense of buying a physical fax machine. Faxing from the Mac also eliminates the silly and wasteful ritual of printing something out just so you can feed it into a fax machine. And because your fax originates directly from the heart of Mac OS X instead of being scanned by a crummy 200-dpi fax-machine scanner, it blesses your recipient with a great-looking document.
Here's the basic idea: When faxes come in, you can read them on the screen, opt to have them printed automatically, or even have them emailed to you so that you can get them wherever you are in the world. (Try that with a regular fax machine.) And sending a fax is even easier on a Mac than on a regular fax machine: You just use the File→Print command, exactly like you're making a printout of the onscreen document.
There are only two downsides of using a Mac as a fax machine: