I think the logic behind it is that if you're on shared hosting, and a site on the same IP address as yours is sending out spam emails or phishing, you're site could also get blacklisted. To avoid this, you can get a dedicated IP address, meaning you don't get confused with other spammers, phishers etc.
Of what purpose did you have in mind when you purchased one? If you have money transfers on your site or want to use anonymous FTP then you'd have a good purpose for one.
Sometimes it comes as a package deal. It did for me. I have them. I think it's more than worth it just to avoid having your IP tainted by someone else's mucking around like DarkRaven said.
