In addition to allowing the specification of backup exchangers, observe that not every domain has its own mail server, so it is necessary to be able to specify a mail server which exists on another domain as authorized to exchange mail so that administrative and system messages addressed to postmaster, root or any technical/administrative contacts listed in the DNS WHOIS records can be delivered, even if they do not exist on the current domain.
You just don't need that for ftp and http because those services do not initiate outbound connections like MX nor are they considered official points of contact.
