All those commenting that "oh it took me 10 min to get used to the interface, bla bla bla".. 90% of every other program has yet to adopt this "ribbon interface" so for all I care MS just did this to be different.
The question is why. There was no need to change it.
Honestly, I can't even think of a single other program that had menus nested as deeply as Office 2003 that were core functionality. Even other complex programs like 3DS Max, Blender, and Photoshop didn't have menus nested as deeply that you need to use on a regular basis.
MS changed it because what Office evolved into was a shitty UI that was very hard to navigate and even harder to discover new features. The ribbon largely solves that. There may not have been a "need" to change it, but you also don't "need" a faster computer, or a prettier UI, etc... They changed it to make it better, and IMO they succeeded at that.