If you have more than one website or ever plan on expanding your web presence, multiple domain hosting is a must. As its name implies, the service lets you host multiple domains on a single account.
Believe it or not, some shared hosts actually limit you to one domain. Multiple domain hosts cost the same and do not have this restriction. It’s all a matter of policy, really.
Keep in mind that all space and bandwidth usage are computed for all your sites as a whole, not individually. Any domains you add will be hosted on the same IP address, which shouldn’t be a problem. Personally, I wouldn’t choose any host that only allowed me to have one domain. It’s your hosting account, why not use as much of it as you can?