I was wondering the same thing. Doing this course after the HTML/CSS ones has mostly been review but display: flex; is the first topic that wasn’t covered in the CSS course at all - we only learned about other display values such a inline, block, and inline-block, none of which seem to have any impact on this page when used instead of flex.
I did some experimenting and it looks like the key difference has to do with parent/child element relationships.
HTML:
<!DOCTYPE html>
<html>
<link href="./test.css" type="text/css" rel="stylesheet">
<!--TESTING SPACE-->
<div class="rectangle">
<p>I'm a rectangle!</p>
</div>
<div class="rectangle">
<p>So am I!</p>
</div>
<div class="rectangle">
<p>Me three!</p>
</div>
<div class="parent">
<p>I'm the first child!</p>
<p>I'm the second!</p>
<p>I'm the third!</p>
</div>
<!--TESTING SPACE-->
</html>
Changing display to flex for the “rectangle” class had no effect, so that tells me that flex is only useful on container elements. On the other hand, changing display to inline-block for the “parent” class created some strange layout changes that I can’t explain.