It allows you to separate non-stateful business logic from the stateful update logic. Reactive frameworks usually handle the state mutation for you, eliminating the potential for UI glitches where something that depends on the updated state doesn't get recomputed/redrawn.

A lot of “dependency injection” type frameworks run this way.

The framework or runtime calls your function, and the return value tells it what to do next.

Prominent examples include React and Excel.