An IP address is a unique number which distinguishes a website or a server on the Internet, so if you have a dedicated IP, it shall be used solely by your sites and won't be shared with others as it happens with shared website hosting accounts. In case you have your own web server, you shall have a dedicated IP, but you may require additional ones for different purposes. If you have an Internet store, for instance, you shall need an SSL certificate for it, to make sure that the payment information your customers submit shall be encrypted and safe. 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 which visitors input to be secured. The SSL certificate requires a dedicated IP, which needs to be different from the one which you already have on the hosting server. You might also need a separate IP for an app such as a VoIP server, or if you need a slightly better functionality for a certain site, which may influence its position in search engine results.