(Yii2) Why separate backend into Backend and Frontend?

Question

Yii

I started to get acquainted with Yii2 and I just can’t understand why there is a division into backend , frontend and common in yii2-app-advanced . What exactly is in each of these folders and how are they related? What kind of projects should use this division?

Answer 1

We have a similar structure, for the first version of the
backend framework - the admin panel and everything for it.
frontend - the public part of the site.
common - common extensions\components\models.

Answer 2

What's the point?
Why can't you just deny access to everyone, except for exceptions, at specific urls like /admin/?

Answer 3

backend - server side of the site
frontend - front end (html, css, js)