Nobody will explain you like this, but the main point was being able to satisfy the compiler without introducing an escape hatch into the language.

Haskell is based on Miranda, and Miranda is based on Hope. Purely functional languages were really purely functional, academic experiments with no way to express side effects, so no way to express practical programs.

Philip Wadler took the monad (the name that already existed in category theory), and showed how computations could be expressed in Haskell with the “do notation” as an example. That made Haskell practical without breaking the “beauty” of the language, by having to introduce new special syntax or something outside the type checker capacity.

So, I don’t think there’s a motivation besides being an exercise in expressivity within the limitations of pure functional programming. Similar ideas in describing computation as lazy executed instructions already existed elsewhere, like the interpreter pattern.