An IP address is a unique number which separates a site or a server on the world-wide web, so if you have a dedicated IP, it'll be employed only by your websites and won't be shared with other people as it happens with shared web hosting accounts. If you have your own server, you'll have a dedicated IP, but you may require additional ones for different purposes. Provided you have a web-based store, for instance, you shall need an SSL certificate for it, in order to ensure that the payment information your clients submit shall be encrypted and secure. The same is valid in the event that you have a login form of some sort and you would like the usernames and the passwords that visitors input to be protected. The SSL certificate requires a dedicated IP, which ought to be different from the one which you already have on the web server. You might also need an independent IP for an application such as a VoIP server, or if you want a slightly better overall performance for a certain site, which may affect its position in search engine results.