I'm just learning PHP, and I'm confused about what the purpose of the __construct() method?

If I can do this:

class Bear {
    // define properties
    public $name = 'Bill';
    public $weight = 200;

    // define methods
    public function eat($units) {
        echo $this->name." is eating ".$units." units of food... <br />";
        $this->weight += $units;
    }
}

Then why do it with a constructor instead? :

class Bear {
    // define properties
    public $name;
    public $weight;

    public function __construct(){

        $this->name = 'Bill';
        $this->weight = 200;
    }
    // define methods
    public function eat($units) {
        echo $this->name." is eating ".$units." units of food... <br />";
        $this->weight += $units;
    }
}