The question, why we use CSS in web designing is very important because it answers tell us why we use CSS in web designing.
CSS is used in web designing for the styling web page. With the help of CSS, we provide style to HTML elements like background color, font-size, font-color, height, width, etc. CSS is used to define the layout of the web page, we can't imagine a proper web-page layout without CSS. See some point listed below.
CSS used in web designing because -
It helps to define the structure of the web page
It helps to organize the layout
It is used to styling HTML elements like <p>, <div>, <h>, <ul>, <li> etc.