To the extent that I have 'caught' it and understand and practice it, it is because to me it is a usability issue. Not in the sense that I believe usability should be extended to the widest possible segment of users (although I do) but because I tend to hack on things, and the more accessible a thing is, the more it is hackable. When you separate the semantic markup from the frippery, you are making it more hackable, in addition to making it more usable.

I mean, I would hopefully also be thoughtful and not put in a seizure inducing gif, but that is always my second "oh wait. crap." When I am really practising accessibility as a primary concern, it is because to me the accessible structure is more beautiful/hackable to me.

(self-involved!)