What is the purpose of CSS in HTML web page?

As a web designer, we should very well understand that why we are using CSS and what it's important / role in HTML. You can imagine HTML elements like bricks and CSS properties like cement when we combine both we can make a beautiful building. With the help of CSS we can style HTML tags for example - for <p></p> tag we can use different-different CSS properties like font-size, color, text-align, etc. like <p> tag we can style all tags in HTML with the help of CSS.