An IP address is a unique number which distinguishes a site or a server on the Internet, so if you have a dedicated IP, it'll be employed only by your Internet sites and shall not be shared with other people as it happens with shared web hosting accounts. If you have your own web server, you shall have a dedicated IP, but you may need additional ones for a variety of purposes. In case you have a web-based store, for example, you will need an SSL certificate for it, so as to make sure that the payment info your customers submit will be encrypted and secure. The same is valid in case you have a login form of some kind and you would like the usernames and the passwords which visitors type in to be secured. The SSL certificate requires a dedicated IP, which should be different from the one which you already have on the server. You may also need an independent IP for an app such as a VoIP server, or if you would like a slightly better efficiency for a given website, which shall influence its position in search engine results.