Projector shutters: http://en.wikipedia.org/wiki/Movie_projector#Shutter
23.976 is not just faster to render, it’s a better quality image at any given data rate, more data per frame. So it’s faster, better quality and cheaper. Motion blur in AE has very little penalty and you will want it on 29.97 as well.
Also, DCP format is still stuck at 24.000fps for universal standard and 29.97p to 24 fps looks like dirt regardless of your budget. While 23.976 to 29.97 is a visually seamless conversion.
If you real need smooth motion/live feel, skip 29.97 and go 59.94p. That is truly smooth and big difference over 24fps feel.