Why redirect to index.php file after a submit custom-contact form?

<?php
//Setting the Content type. Basically telling the browser what type of content it is dealing with.

if ($_SERVER["REQUEST_METHOD"] == "POST") {
        // get the posted data
        $name = trim($_POST["name"]);
        $email_address = trim($_POST["email"]);
        $subject = trim($_POST["subject"]);
        $message = trim($_POST["msg"]);

        // check that a name was entered
        if (empty ($name))
                $error = "You must enter your name.";
        // check that an email address was entered
        elseif (empty ($email_address)) 
                $error = "You must enter your email address.";
        // check for a valid email address
        elseif (!preg_match("/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,4})$/", $email_address))
                $error = "You must enter a valid email address.";
        // check that a subject was entered
        elseif (empty ($subject))
                $error = "You must enter a subject.";
        // check that a message was entered
        elseif (empty ($message))
                $error = "You must enter a message.";
        // the field named address is used as a spam honeypot
        // it is hidden from users, and it must be left blank
        elseif (!isset($error) && $_POST["address"] != "") 
                $error = "Your form submission has an error.";
        // this code checks for malicious code attempting
        // to inject values into the email header
        elseif (!isset($error)) {
                foreach( $_POST as $value ){
                    if( stripos($value,'Content-Type:') !== FALSE ){
                        $error = "There was a problem with the information you entered.";
                    }
                }
        }       

        // check if an error was found - if there was, send the user back to the form
        if (isset($error)) {
             // Set a 400 (bad request) response code and exit.
             http_response_code(400);
             echo '<span class="icon-sad"></span> Oops! Hubo un problema con el envío. Por favor, completa el formulario y vuelve a intentarlo.';
             exit;
        }

        // Set the recipient email address.
        // FIXME: Update this to your desired email address.
        $recipient = "prueba@prueba.com";

        // Build the email content.
        $email_content = "Name: $name\n";
        $email_content .= "Email: $email_address\n";
        $email_content .= "Message:\n\n$message";

         // Build the email headers.
     $email_headers = "From: $name <$email_address>";

        // Send the email.
        if (mail($recipient, $subject, $email_content, $email_headers)) {
                // Set a 200 (okay) response code.
                http_response_code(200);
                echo '<span class="icon-happy"></span> Gracias! Su mensaje ha sido enviado.';
        } else {
                // Set a 500 (internal server error) response code.
                http_response_code(500);
                echo '<span class="icon-sad"></span> Oops! Algo fué mal y no se pudo enviar tu mensaje.';
        }
} 
?>


<?php get_header(); ?>
</div> <!-- Header container tag -->

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

    <?php the_content(); ?> 

  <div class="page-section hide-mobile">
    <h1 class="title"> <?php the_title(); ?></h1>
    <h3>Envianos un correo electrónico, nos encantaría saber de ti</h3>
  </div>

  <div class="container contact-form">
    <div class="grid-8 form">
      <form id="eform" role="form" method="POST" action="<?php the_permalink(); ?>">
        <h3>Todos los campos con un * son obligatorios.</h3>
        <div class="form-group">
          <h4><label for="name">Nombre*</label></h4>
          <input id="name" type="text" name="name" required>
          <span class="validation">Por favor introduce tu nombre</span>
        </div> 

        <div class="form-group">
          <h4><label for="email">Email*</label></h4>
          <input id="email" type="email" name="email" required>
          <span class="validation">Mejor si el email es el tuyo, gracias</span>
        </div>

        <div class="form-group">
          <h4><label for="subject">Asunto*</label></h4>
          <input id="subject" type="text" name="subject" required>
          <span class="validation">Necesitamos saber de que se trata</span>
        </div>

        <div class="form-group">
          <h4><label for="msg">Mensaje*</label></h4>
          <textarea id="msg" name="msg" rows="10" required></textarea>
          <span class="validation">No lo dejes en blanco, escribe algo!</span>
        </div>

        <div class="form-group" style="display: none;">
          <!-- the field named address is used as a spam honeypot --> 
          <!-- it is hidden from users, and it must be left blank -->
          <h4><label for="address">Address</label></h4>
          <input type="text" name="address" id="address">
          <p>Humans: please leave this field blank.</p>     
        </div>

        <input id="submit" name="submit" type="submit" ></input>
        <input type="hidden" name="save" value="contact">

        <div id="form-messages"></div>

      </form>
<?php endwhile; endif;?>

<?php get_footer(); ?>