Why $_SESSION[$alert] is unset in OOP Login System

[Drew2]

Hello you wonderful coders you.

I've been staring at this line of code (line 74 highlighted below) for a while in m_template.php, and want to share what it's doing.  

function getAlerts()
    {
        $data = '';
        foreach($this->alertTypes as $alert)
        {            
            if (isset($_SESSION[$alert]))
            {
                foreach($_SESSION[$alert] as $value)
                {
                    $data .= '<li class="'. $alert .'">' . $value . '</li>';
                }
                unset($_SESSION[$alert]);
            }
        }
        return $data;
    }

The unset is here so that the next time a user clicks the submit button, the $_SESSION variable will have a clean slate (at least as far as alerts are concerned).  Otherwise, a user might click the submit button when they've entered valid information, and receive an error message in the members page. 

I just commented out the unset line and ran the code.  First I entered wrong information and clicked submit.  So $_SESSION now has this in it "you entered an invalid username or password."  Then, I entered valid information (so now $_SESSION should also have this in it "You successfully logged in to the members page").  When I did that, I was redirected to the members page.  At the top were the alerts.  There was an alert saying "You successfully logged in to the members page" and then the error: "you entered an invalid username or password."

Just thought others may have been confused like me about why that line of code is in there at all.