Why XCode needs to be stopped to update iTunes ? I'm wondering everytime iTunes has an update : 

Why XCode needs to be stopped to update iTunes ? 

Is it some kind of shared component ?
Both depend upon the same PrivateFramework.

Specifically usbmuxd is part of the MobileDevice.framework which communicates to iOS devices. When an iTunes update includes an update to usbmuxd, then Xcode can be flagged to prevent it from interacting with the subsystem while the update is being performed.