The pictures say it all, but I'll add more.... in the first picture, the vertical offset plane aligns with the grid so anywhere else on the sketch it will snap to the grid but with the green line/X (words escape me) it won't snap to that grid line. In the second picture, the offset plane is not aligned with the grid and also cannot be snapped.
I'll work around it but it'd save me time being able to use offset planes for snapping references in sketches since the planes go through multiple components I want to modify along that reference.

