what's the use of <header>?

Answer 1

the header tag used for top of the page website logo ...

Answer 2

in html4 we would have to create a <div id="header"></div> in html5 they introduced <header></header> you can clearly see why this was done and now you know it's use. there is also common tags like <nav></nav>. <main></main>. <footer></footer>. many more can be found here :). https://www.w3.org/TR/html5-diff/