Whenever you can make a glyph by scaling/stretching/rotating a component. Of course, it’s all font-specific, but there are cases when a mere mechanical transformation is enough:
• endash > emdash
• period > bullet
• period > ellipsis
• paren > paren.case
• dotaccent > dieresis
• accents > accents.case
• accents > accents.narrow
• Ľ ľ and ť sometimes may need a subtle scaling of the accent
• comma > quote marks
• geometric shapes and modular fonts
Because transformed components don’t seem to be well supported in variable fonts (especially, if there are different transformations in different masters)