Question

I mean, If all element is block-level, then there may be more simple to learn. so, why we need this kind of box/element?

Answer 1

If all tags would be treated as block level elements, the following snippet:

<div>This is a <a href=".">link</a>, you can <b>click</b> it.</div>

would look like this in your browser:

The corresponding code:

<div>This is a <a href="." style="display:block">link</a>, you can <b style="display:block">click</b> it.</div>

Edit: By the way, if there is a need for you to deal with block elements only, just reset the HTML using CSS: * { display:block}