At uninstall time everything in the registry table, that is in a non-permanent component, will be removed.
You should populate the RemoveRegistry table if you have a registry entry that is not in the registry table. What does that mean in practice? If an application creates a reg entry after install time then the installation and the registry table will know zip about it. However, you, as a living, thinking human being, might know that it will have been created and you might want to remove it. Enter our new friend the RemoveRegistry table. It lets us remove entries not created by the installation.