The Registry is a heart and soul of any Windows system. It contains information that controls how your Windows appears and how it behaves. Most applications today use registry to store configuration and other important data. When you install an application, a new registry entries will be created. These entries will automatically be deleted when you uninstall the application. Unfortunately, it does not always work that way. Sometimes, you will find that some applications fail to remove their own registry entries. These entries will become obsolete.
After a long period, after installing and uninstalling a lot number of applications, your Windows registry will contain a large number of obsolete entries. These will significantly increase the registry size and thus will slowdown your computer, because Windows will need more time to load, search, and read data from registry.

To keep your computer in top performance, it is recommended to periodically clean your Windows registry.