IG was first developed for "compact" drivetrains, and the sprocket teeth were shaped to more-aggressively transport (shift) the chain at lower chain speeds.

The IG chainrings are marked so that you won't use a non-IG chain, exactly as it says.
The IG cassettes did have thicker cogs (more-pronounced shift ramp features on both sides of each cog) with thinner spacers, so the narrower IG chain wouldn't be so likely to rub against the adjacent larger sprocket, particularly during downshifts.