How HTML elements will display on the web page, paper, screen or in other media, this will be described by CSS.
You can apply the same layout styles using CSS one page to the multiple web pages at one time. So this will save a lot of time and work.
We can store the external stylesheets in CSS files.
Why we should use CSS?
We can define the professional styles for our web pages, including the layout, variations of multiple displays on different devices, screen resolutions, and design. Therefore we should use CSS for our Web Pages.