Simple reason. In order to connect guy on the other end has to know your IP address. If it is dynamic and changes every 12 hours, then guy on the other end has to find out your new IP address every time he wants to connect.
There are dynamic DNS services, dyndns being one of them, which assign URL names to your dynamic IP, so that guy don't have to type new IP address each time he wants to connect, but instead can type in the URL.
So you don't have to have static IP address in order to use remote desktop.
Now. IT people and especially manipulative bosses, could say to you that static IP is needed for remote desktop, period. They are wrong, and could be motivated by many different reasons, ignorance about the truth being least likely of those reasons.
