After Effects: why set posterize time to 12 fps in 24fps comp???

I'm not sure. Most cartoons that were created with hand-painted animation cells, like Merry Melodies, Daffy Duck, Bugs Bunny, and The Road Runner, were animated at 12 frames per second for most movement. The Animation Camera operator took two images of each cell, and the movie was projected at 24 frames per second, which is the standard for Motion Picture projection in all countries with 60 HZ power (most countries). Posterizing time to 12 fps would give you the same thing: two duplicate frames when rendered at 24 fps. 

I don't think that is necessary unless you are sending your rendered footage to a streaming service that does not support 12 fps playback.

If you really want to duplicate the look and the duplicate frames, you can animate your shot in a 12 fps composition, open the comp settings (Ctrl/Cmnd + k), select the Advanced tab, then set Preserve Frame Rate when nested or in the Render Queue. You could then drop that 12 fps comp in a 24 fps render comp and add a moving background that would be less susceptible to judder. That's what a lot of cell animators did when they used multi-plane animation cameras with one layer for the animated characters and other layers for larger foreground and background plates. That would be a better solution than using Posterize Time, and it would render faster.