Returning output vs echoing output issue

<?php
$base_path = $modx->getOption('core_path').'MODXComponents/studentcentre/trunk/core/components/studentcentre/';

$f = $base_path.'snippet.scShowAssignments.php';
if (file_exists($f)) {
   include $f;
} else {
   $modx->setLogTarget('ECHO');
   $modx->log(modX::LOG_LEVEL_ERROR,'scShowAssignments not found at: '.$f);
}

return;

<?php
/*
*
* @package studentcentre
*
*/

$path = MODX_CORE_PATH . 'components/studentcentre/';
$result = $modx->addPackage('studentcentre', $path . 'model/', 'sc_');
if (!$result) {
  return 'Failed to add StudentCentre package';
}

// get the logged in student id
if ($modx->user->get('username') != '(anonymous)') {
    $studentId = 36; // $modx->user->get('id');
} else {
	$url = $modx->makeUrl(55,'','','full'); 
	$modx->sendRedirect($url);
	return;
}

$output = '';

// get the collection of student assignments for that student
$c = $modx->newQuery('scStudentAssignment');
//$c->prepare();
//echo $c->toSQL();
$c->where(array(
	'student_id:=' => $studentId
));
$c->sortby('status','DESC');
$assignments = $modx->getCollection('scStudentAssignment', $c);

if (count($assignments) > 0) { // if the array is not empty...

	// for each assignment, output it's properties along with a link to the assignment
	foreach ($assignments as $assignment) {
		$fields = $assignment->toArray();
		$assignmentUrl = $modx->makeUrl(2658,'',array(
			'aid' => $assignment->get('id')
		));
		$fields['url'] = $assignmentUrl;
		$output .= $modx->getChunk('csAssignmentRowTpl', $fields);
	}
	
} else { // if there are no assignments
	$msg = $modx->getChunk('csMessageTpl', array(
		'class' => 'msg',
		'text' => 'Sorry but there are no assignments available to you yet.'
	));
	return $modx->setPlaceholder('csMessage', $msg);
}
echo $output;
return;

return include $f;

$modx->setPlaceholder('csMessage', $msg);
return '';