Well, I was pretty clear on what it can be used for. So, I'll explain a bit about what it means to have more than one IP and the 'special' IP's.
Let's say your computer has a network card on a local network. This network card may have the IP of 192.168.1.3 for example. However, your LAN doesn't have access to the internet, so you have to use a 56k dial-up connection. When you get on the internet, you now also have an internet IP given to you by your ISP. Let's say it is 12.345.6.78. These IP addresses give your applications special capabilities when they listen on a port. You see, when somebody tries to connect to your computer, not only is there a PORT it's connecting to, it's also connecting to an IP. Here, I'll explain.
You have a FTP server listening on 192.168.1.3:21. In order for a client to connect to it, the destination IP address must be 192.168.1.3. That is an IP only accessible from your network. That means ONLY people on your LAN can connect to the FTP server. If somebody tried to connect to 12.345.6.78:21 - it will be a connection refused cause there's nothing listening on that IP.
The opposite is true. Servers listening on 12.345.6.78 cannot be connected to from other computers on the LAN (unless you have some static routing going on - but that's special). However, it can easily be connected to by people on the internet.
Now, there is also a special IP 127.0.0.1. If a server is listening on that IP, only programs on the same computer can connect to it. If some other computer tried connecting to 127.0.0.1... well, it won't be your computer it's trying to connect to. Understand? That means you can have servers on certain IP's to restrict who can connect to it.
Another special IP is 0.0.0.0. This is where a server is listening on ALL IP addresses. It doesn't matter if you're connecting to 127.0.0.1, 12.345.6.78, or 192.168.1.3 - they'll all successfully connect.
The BIND method is how you can tell the winsock control on WHICH IP you want to listen on. If you don't tell it what IP you want it to listen on, it will always listen on 0.0.0.0 - and you may not want that.
Now go ahead and read what I said about the BIND method above and you will better understand what I'm talking about.