You can get Bahamut from Zero from the huge blue materia if you have Bahamut and Neo-Bahamut.

You can use the red huge materia to combine a full set of mastered summon materia (one mastered of each type) into one compact orb that has every summon on it.

You can use the green huge materia to combine a full set of mastered magic materia into a master magic orb.

You can use the yellow huge materia to combine a full set of mastered command materia into a master command orb.