The DynIP application was designed to be a simple program that will allow you to post your dynamic IP address up to your web site via a FTP connection. This is the case for most internet users who connect to the internet through an ISP which assigns them a different IP address each time they connect.
Making the IP address of your machine widely available, allows other people on the internet to connect to your machine as if your machine was like any other permanent site on the internet. This allows people on the Internet to connect to your web and FTP server running on your local machine. It is also useful for people who are running simple internet chat programs which do not use a permanent Internet server for registration.
You should be aware that by making your IP address public, leaves your machine open to the same security risks normal internet sites have to cope with, so make sure you are aware of the security issues first.