If I am not using <header>, than also I am getting same result.... so what is use of header?

Answer 1

Inside the body tag you can theoretically use divs only but the readability and management will be horrible.
Different tags are useful to make your own CSS framework adding your style to different elements.

The header is used commonly to represent a set of navigational links or introductory content of your page/site.

Answer 2

for example you will use a background color only for the header or for body