I'm not a fan of React (and not use it at work), but imagine that you have multiple login forms, either to same service or to different ones. With React (or any similar JSX framework) you can embed such form component in your application = it will behave the same way in any place you put it in (or can be customised dependent on the parameters). Then it can also be tested that it behaves in a certain way and conforms to business rules.

How would I do the same with plain HTML?