UAC is designed to protect your computer from security threats, which is what Windows XP was criticized for before Service Pack 2. It is an additional layer of security to protect you. Just pressing OK after you run a program is a lot better than having a malicious program destroy the entire OS.

Windows is used for many different reasons and in many different environments. Some people use it for things that are very critical. What if you lost years of work?

So this is a workaround for people who don't have a server, don't know how or don't want to spend time configuring local and group policies on their PC.