> why did the presenter show Unity through screenshots rather than a live demonstration of it?
Because it makes giving the presentation a lot easier?
IMO there is a difference between 'GUI demo' and talking about a GUI. By using screen shots he can be /sure/ he will be able to follow the presentation order he wanted to, will not forget anything (no need to double check a list), and will be kept from digressing (bugs cause digressions).
Slides also force you to actually prepare the talk. Many of my co-workers assume that they will 'have the whole picture in the right order' in their heads, and spend presentations browsing for things and trying to remember what else they wanted to mention. In the process they also put 10x more information on screen than what they should.