There are basically two types of IP addresses:
1) Static IP address
2) Shared IP address
An IP Address is a 32-bit number that identifies a computer on the Internet. Every web site on the internet is found not by its domain name but by its IP address.
When someone types the address: www.xyz.com it is translated into an IP address and then the computer is directed to that IP address which is the web site.
Now every single website has an IP address specifically allocated to it. If every site assigned a separate IP address then there could be a problem with running out of IP addresses so a lot of the sites on the server use a single IP address for multiple sites.
Thus, using more than one site on an IP address is called a Shared IP address. If a site has its own IP address, and shares with no one else, it is called a Static IP address.
You can always access a site which has a static IP address by using its IP address alone, but you can not access a site using a shared IP address by typing in the IP address alone because when you type in a shared IP address you arrive at the server but the server does not know which site you are looking for as you have not told it which domain name you want.
Requirement of static IP address:
1) The main reason to have a static IP address for your site is that, you can use SSL encryption on a static IP address only. In order to transmit sensitive data over the internet, it must be encrypted to prevent someone from intercepting the information.
2) In case if the site needed the anonymous ftp access (anyone can download files of the site), the site needs to have a static IP address.
Other than these two reasons there is no need for a site to have its own IP address.