what is the evil demon thought experiment and what was its purpose?

In thought experiments philosophers occasionally imagine entities with special abilities in order to pose tough intellectual challenges or to highlight apparent paradoxes. Examples include:

Descartes’ malicious demon – Cartesian skepticism (also called methodological skepticism) advocates the doubting of all things which cannot be justified through logic. Descartes uses three arguments to cast doubt on our ability to objectively know: The dream argument, the deceiving God argument, and the malicious demon argument. [1] Since our senses cannot put us in contact with external objects themselves, but only with our mental images of such objects, we can have no absolute certainty that anything exists in the external world. In the evil demon argument Descartes proposes an entity who is capable of deceiving us to such a degree that we have reason to doubt the totality of what our senses tell us.
Laplace's demon – A hypothetical all-knowing entity (later called "Laplace's Demon") who knows the precise location and momentum of every atom in the universe, and therefore could use Newton's laws to reveal the entire course of cosmic events, past and future. Based upon the philosophical proposition of causal determinism. (See also causality).
Maxwell's demon – A demon able to distinguish between fast and slow moving molecules. If this demon only let fast moving molecules through a trapdoor to a container, the temperature inside the container would increase without any work being applied. Such a scenario would violate the second law of thermodynamics (see also Epicurean paradox#Epicurus).
Morton's demon – A demon who stands at the gateway of a person's senses and lets in facts that agree with that person's beliefs while deflecting those which do not. Used to explain the phenomenon of confirmation bias.

Descartes’ malicious demon –...
Since our senses cannot put us in contact with external objects themselves, but only with our mental images of such objects, we can have no absolute certainty that anything exists in the external world. In the evil demon argument Descartes proposes an entity who is capable of deceiving us to such a degree that we have reason to doubt the totality of what our senses tell us.