When your character casts a spell, you get a pretty sequence of colors swirling around their hand depending on the school the spell cast came from. A necromantic spell, for example, looks quite different from creating an illusion. The Baldur's Gate and Icewind Dale games use different sets of animations for the schools. The Icewind Dale Casting Graphics component allows you to use the Icewind Dale graphics on the Baldur's Gate series of games, whereas the Baldur's Gate Casting Graphics component will do the reverse. Note that this is purely a cosmetic change--the spells themselves function exactly as before.