Html5 structure

Question

I want to ask about the page structure elements ( header, footer, article and so on)
Did their task just define the page? If yes!
That means they are not important to use or there is something I don’t know.

Answer 1

Semantic markup means that HTML tags are never chosen based on the way they appear in a web browser — they're chosen based on the importance and structure of the content. ... The goal is to make it easier for browsers, developers, and crawlers to distinguish between different types of data.

For the use of the whole structure give a look at this:
https://dev.opera.com/articles/new-structural-elements-in-html5/

Answer 2

Every section is their importance even some companies keep different people for different jobs like Some will work on article some will handle Comments on page etc so each and every section has their own importance thats how successful companies work in a team