What does this do?

This is about the first project demo: https://codepen.io/anon/pen/vqJoJO

In the media query for #main, what is the purpose of making the margin 0 for the specified width range?

This media query is executed when we have not so much place to output our code in browser, here max-width: 460px; and we want to have more space for main content, instead of margins and other similar things.