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To make the content immune to DMCA takedown requests.

This also protects the account itself from suspension.

Not sure if this is the primary reason, but it should be enough to throw off anti-spam systems that are looking for repeated posting of the same content within a short timespan

I imagine copyright plays a role.

Maybe, but why would a content farm care about copyright? Does Facebook have a photo-equivalent of YouTube's Content ID?

If the content farm is using images subject to copyright then they could lose their page with just a couple of image reports for copyright infringement, and their hard work would be for nothing.