When would html lang be ever useful?

I wonder? or do I?

This is a simple html page I have:
<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
    <style type="text/css">
        div {
            font-size: 64px;
        <div style="text-transform: uppercase;">

And what I will see in the browser is:

Ok, so what is wrong about this? Well, lets change the lang to tr and try..
<html lang="tr">

And reload:

Ah, I see..