just as the subject states........

There generally isn't one, aside from being able to use private SSL, since you can only have one SSL certificate configured per IP address, and for creating your own anonymous FTP site, since FTP does not support host headers like HTTP does. If you've got a dedicated server, having multiple IPs allows you to bind different services to the same port on different IPs, like simultaneously running IIS and Apache on Windows off of different IP addresses on port 80.

Having the same IP address as a reseller , as other resellers isnt this a risk to anonymousity?

In my experience, most companies that offer reselling will give you 3 IP addresses. One for the reseller account and the other two for the nameservers.

Having said that, some will offer a cheap introductory reseller account on their own servers, so yes that would give the game away. :)

As a reseller, if the IP that is given to you is different from the web host, it would give you a sense of anonymity.