At this point, I think artists who publish pages and pages of transformation sequences with barely-perceptible changes are just trying to pump up their page counts.

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I think part of it is the limitations of e-hentai's format. Seen individually, each picture is almost indistinguishable from its fellows, but when you view them quickly and seamlessly in sequence then it takes on an effect similar to that of a flipbook.
Animation for cheap, essentially.

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