Rails can automatically create what you need to create in common when developing any web application. It is one of the web application frameworks.
The web application framework has the same role as the "mail template statement" that you use every day. Both are the same in that they are used to streamline work. The email template statement is used each time a new email is created. The web application framework is used every time you develop a web application.
Ruby on Rails implements a feature called Asset Pipeline. In a nutshell, its role is "a function that makes coloring on Web screens easier."
As a metaphor, it is called "coloring", but specifically, it is to write and create CSS, JavaScript, and Image files that are linked to HTML so that the Web browser can recognize the screen design.
Beginners struggle to learn the role and directory structure of the Ruby on Rails framework. Therefore, it is easy to overlook the "mechanism of displaying the screen on a Web browser".
Most of the information on the Internet is viewed via a web browser. The screen is displayed on the web browser, "Many beginners proceed with learning Ruby on Rails without recognizing that the web browser needs to recognize HTML, CSS, and javascript.  What is displayed on the web browser is recognized as being drawn by linking HTML, CSS, and javascript. This mechanism is being realized by the asset pipeline.