Types of Web Fax Services

Inbound or Outbound - most people only use one of them.
Fax providers use 'leased lines' - just like cell phone providers. The pricing behind leased lines is that it's more expensive to send 'outgoing' faxes than to receive 'incoming' faxes.
So, you will see web fax plans available with differing usage limits.
Today, most users are just 'receiving' faxes. i.e.; a signed contract, or an order from a distributor.
Outbound faxes have largely been replaced with e-mail blasts.

Can't believe I still fax things...

I remember when my company got one of the first fax machines commercially available. Very few companies had them yet and we used it just to send reports to the company headquarters. This technology, for sure, was going to put FedEx out of business.
Then, as soon as other local companies started getting machines the junk faxes started. First it was joke lists. Then it was advertisements. Spam was invented.

Today, the humble fax machine is still in every office. Still providing something that personal computers can't yet do; getting signatures on paper.

However, with the advent of email fax services, web based e-mail faxes actually run through spam filters.