It's got the benefit of making windows updates smaller. I've read that it will reduce updates very significantly... can't remember where I saw Microsoft blogs of development groups that tout it's effectiveness but some big updates get reduced by 60 or so MB
 here's a decent link http://msdn.microsof...28VS.85%29.aspx
Windows Installer 4.5 and later:
Beginning with Windows Installer 4.5, developers can mark components in a patch with the msidbComponentAttributesUninstallOnSupersedence value in the Component table. If a subsequent patch is installed, marked with the msidbPatchSequenceSupersedeEarlier value in its MsiPatchSequence table to supersede the first patch, Windows Installer 4.5 and later can unregister and uninstall components marked msidbComponentAttributesUninstallOnSupersedence to prevent leaving behind unused components on the computer. If the component is not marked with with this bit, installation of the superseding patch can leave an unused component on the computer. Setting the MSIUNINSTALLSUPERSEDEDCOMPONENTS property has the same effect as setting this bit for all components.
Edited by jawz101, 24 December 2009 - 06:09 PM.