Comment by Chinjut

The significance of monads in programming is entirely because of the adjunction between the category of types and "pure" functions and the category of types and "monadic" functions (i.e., the Kleisli category of the monad). A monad is used to encode the latter in terms of the former via the Kleisli adjunction (whereby monadic maps from FX to FY correspond to pure maps from X to GFY, with the monad GF thus arising from an adjunction F -| G [where F can be taken to be identity on objects in the case of interest]).