well varying speed only or extrusion rate only or both speed and extrusion rate we can influence the visible pattern perimeters give to our prints ...
e.g. this is what you get by varying extrusion rate only (available in another commercial slicer as paid feature), it's ideal to e.g. produce a wood grain effect on the print:
on the other hand just changing speed (also called "Velocity Painting") is perfect for vase mode objects that are transparent or semi transparent, would probbly do wonders for lithophanes too... (lot of info here, postprocessing software etc.. https://www.velocitypainting.xyz/blog/ )