Functional Components Lifecycle
Class components live as long as the class instance lives, so the state is preserved between the subsequent re-renders.
Functional Components live as long as the rendering takes time. There is no instance. Once they get mounted, the component function is deleted (it has fulfilled its purpose).
In that sense, functional components are basically the render functions of the class components. They live JUST during the rendering. The variables are recreated on each render, so it's unable to keep the state, side effects, refs to HTML etc. without using hooks.