The front-end starts with HTML and CSS. Imagine a building; HTML is the skeleton of the building and CSS is the facade and fine work. This is the first essential step.
The buildings require doors, windows, elevators, and electricity to be usable. JS (and it's popular library, React) is like these elements and prepares the site for the user.
Constantly changing the codes and adding content whithin codes are difficult for site administrators. They want to manage everything with a few simple clicks. WordPress is the very content management system.