Sometimes applications are creating files after an MSI installation has completed that are therefore not visible to the MSI setup. These files may need to be removed before install or at least on uninstall to free up disk space and for other cleanup reasons. In WiX Toolkit 3.6 there is a new feature named RemoveFolderEx to solve this problem very easily.
In case that you install or uninstall an application you are able to recursively remove directories without writing strange CustomActions for this task. For install I'm using it to upgrade from a manual "copy" type installation to an MSI version. It's also a must if you do not have any idea about the file names created in the application directories or simply for files left behind for unknown reasons. There are a few requirements if you'd like to use the tag in your WXS scripts.