For example, imagine a company logotype being scaled to some arbitrary size. It needs to still look like the logotype but key elements like fine stroke need to be sharp.
Fonts is another example of this problem, where there are several solutions. The most significant "solution" to make sure text is legible is to align the baseline (the relative zero point) to pixels. This works for text but does not work so well for arbitrary graphics.
@rsms One can put some graphic elements in a fonts (icons, emoji), and hint them.