I guess its the effect it has on certain people, I would bet money that this is not just myself.
Sure there must be some programmers that are such good artists/musicians/coders that every demo looks easy but I think they are a tiny minority.
The majority of people could benefit from watching and thinking about the techniques behind the demos. Undoubtedly it would lead to improvements in their own skill.
I think a good demo not only entertains but it causes a person to question what they thought was possible on the target system.