Was that actually the goal, or was it just a happy side effect? (I don't know the history.)

Was a 2x speedup for dithering even important at the time, especially if it involved a sacrifice of quality? It's not like dithering images was something people did regularly, in the first couple generations of Macs.

You'd dither a few images for a game or something you were building, that you were lucky to get from a scanner. It was a pretty rare thing. Speed wasn't really relevant, as far as I remember.