My take: Monads were created to allow pure functional languages like Haskell to have side effects, particularly input and output, without breaking the pure func

My take: Monads were created to allow pure functional languages like Haskell to have side effects, particularly input and output, without breaking the pure functional syntax. Monads have many other uses, but state and side effects was the original raison d'etre. Haskell, for example, is a pure functional language that doesn't have an explicit syntax for side effects or state. But a language without any side effects can not do anything more useful than heat up the CPU: you have to have input and output, which means allowing for side effects. The Monad was the most elegant solution presented for that issue. In code a monad looks like a function which has more outputs then inputs, and the compiler takes care of channeling the additional input and output between functions adjacent to the monad in way that doesn't break the functional syntax. This differential between function inputs and outputs allows for some interesting use cases and thus the many varieties of monads. Also the compiler makes sure the order lazy evaluations dependent on monads happen in the right order so that you're not trying to parse file contents before you've read the file (because in a pure functional language, like cells in a spreadsheet, everything is intended to be evaluated all at once).