What is the purpose of using "{ }" keys to access the property of a class?

Question

I was doing some tests with the magic method __get PHP and discovered a different way to access class properties, which is by using keys { }, see:

return $this->{$bar};

I can also access the property in the traditional way, which would be:

return $this->bar;

And the obtained result seemed the same to me. Now see a full illustration example:

class Foo 
{
    private $bar;
    private $prop = ":(";

    public function __construct($bar)
    {
        $this->bar = $bar;
    }

    public function __get($bar)
    {
        if ($this->{$bar}) return $this->{$bar};
    }

    public function getProp()
    {
        return $this->{$prop};
    }
}

$obj = new Foo("SOpt");

echo $obj->__get('bar');

Exit:

Sopt

Note that if I use this syntax in a method get traditional getProp():

return $this->{$prop}; 

i get the following error:

<br />
<b>Notice</b>:  Undefined variable: prop in <b>[...][...]</b> on line <b>20</b><br />
<br />
<b>Fatal error</b>:  Cannot access empty property in <b>[...][...]</b> on line <b>15</b><br />

The illustration example can be executed here.

This left me with some doubts that I will address below.

Doubts

1. What is the purpose of using the keys { } to access the class ownership?
2. This way of accessing properties only works with methods magic or can serve other contexts?