Actually moving parallel to it would make the artifacts worse as it'd distort the projection based on distance (i.e. parallax). With a rotation if you're rendering it onto a sphere, doing that rotation will have a very little amount of distortion from it comparatively. It still needs to be rerendered to be correct but this can allow for better intra-frame changes (say game runs at 60 hz, and you need 120hz) with less computation. As a sibling (to you) commenter pointed out, this will make the image slightly "fuzzy" or softer because you end up moving the image on a fractional amount of pixels. As I understand it though, that extra intra-frame can mean the difference between some people getting a VR migraine or not.