This unit's final lesson is about forms.
Think about how forms work in a typical, non-React environment. A user types some data into a form's input fields, and the server doesn't know about it. The server remains clueless until the user hits a "submit" button, which sends all of the form's data over to the server simultaneously.
In a React form, you want the server to know about every new character or deletion, as soon as it happens. That way, your screen will always be in sync with the rest of your application.