The reason for the update is because Windows XP is running out of product keys:
Windows XP Professional with Service Pack 2c resolves a shortage of product keys caused by the longevity of Windows XP Professional, says the company. In order to allow OEMs to ship XP right up to its end-of-life on January 31, 2009. There are no other features or fixes included in the new version.
January 31, 2008 marks the EoL for Windows XP for all channels except System Builders, who have a further 12 months before having to find an alternative OS.