Why is doctype used in html5

Question

I know that in html 5 we have to type doctype before html so why are we doing this is there any reason behind it

Answer 1

it'll tell the browser which version that the page is using. so it could choose which rendering mode that fit with the page
https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode