Era da un po' che me lo chiedevo ma ammetto di non aver indagato più di tanto:

There have been a number of articles on blogs both at Smartphone Thoughts and also CoolSmartphone about the role/purpose of Windows Update.
Let me explain some history on this. We introduced a capability in Windows Mobile 5.0 called Image Update this would allow a Mobile Operator or OEM to update the image of the device without having to do a complete ROM reflash. That capability is taken one step further in Windows Mobile 6 with Windows Update..[...continua alla fonte...] ...
Jason Langridge's WebLog - MR Mobile! : Windows Mobile Update - what is it for?