You fool someone who cast a spell upon you. If you make your saving throw against a spell, the caster and everyone observing see instead an image of you failing it. Thus, you could convince an opposing spellcaster attempting to use charm person on you that the spell succeeded, or that an answer you give under a discern lies is the truth when it is a lie. The spell can also create an illusion that makes it appear to the caster of a damage-inflicting spell that you were killed by the spell when in fact you have made your saving throw and are now invisible as per the invisibility spell. Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion.