Established / Related enabled?

I’m going over VYOS Wiki’s page, and under the configuration of firewall, noticed the following (See attached picture, “Question.png”).

Few questions regarding that:

  1. When declairing a firewall rule, why do we need to set some kind of rules on the “in” part of the interface?. why isn’t the “out” enough?.
  2. can anyone please explain the “Established enable” and “related enable”?.
  3. On one of the rules, I saw he used: state new ‘enable’ => what is it used for?.

Thank you.