An IP address is a unique number which distinguishes an Internet site or a server on the Internet, so if you have a dedicated IP, it will be used exclusively by your sites and won't be shared with other individuals as it happens with shared hosting accounts. In case you have your own web server, you shall have a dedicated IP, but you may require additional ones for various uses. Provided you have an electronic store, for instance, you will need an SSL certificate for it, to make sure that the payment info which your customers submit will be encrypted and protected. The same is valid if you have a login form of some sort and you want the usernames and the passwords which visitors enter to be secured. The SSL certificate requires a dedicated IP, which has to be different from the one which you already have on the machine. You could also need an independent IP for an app such as a VoIP server, or if you desire a slightly better efficiency for a particular site, which may influence its position in search engine results.