Why does WorkElement save work for session user ?

$user = getSessionUser();

...

// If realWork has changed (not through Dispatch screen), update the work
if ($diff != 0) {
  // Set work to Ticket
  $idx = - 1;
  // Will retrive work for current WorkElement, Current resource
  $crit = array (
      'idWorkElement' => $this->id,
      'idResource' => $user->id 
  );

saveDispatchWork.php

ProjectPlanningElement::updateSynthesis

//$element is the element to which it should save work, for exemple a ticket
//$userId is the Resource/User id to which work should be attributed

$startTime = $element->WorkElement->ongoingStartDateTime;
$stopTime = date ( 'Y-m-d H:i:s' );
$workTime = workTimeDiffDateTime ( $startTime, $stopTime );

Sql::beginTransaction();
$saveDispatchMode=true;

$work = new Work();

$work->setDates(date("Y-m-d"));
$work->idResource=$userId;
$work->idProject=$element->idProject;

if ($element->idActivity) {
    $work->refType = "Activity";
    $work->refId=$element->idActivity;
} else {
    $work->refType = $class;
    $work->refId = $id;
}

$newWork=Work::convertImputation($workTime);
$diff=$newWork;
$work->work=$newWork;
$work->idWorkElement=$element->WorkElement->id;
$work->dailyCost=null; // set to null to force refresh 
$work->cost=null;

$ass=WorkElement::updateAssignment($work, $diff);
$work->idAssignment=($ass)?$ass->id:null;

$resWork = $work->save();

if ($resWork) {
    $status = getLastOperationStatus ( $resWork );
    if ($status=='OK') {
        error_log("OK");
        if ($ass) error_log($ass->saveWithRefresh());
    } else if ($status=='ERROR' or $status=='INVALID') {
        error_log("Error when stopping work: ".$resWork);
        break;
    }
}

ProjectPlanningElement::updateSynthesis('Project',$element->idProject);
Sql::commitTransaction();

$element->WorkElement->idUser = null;
$element->WorkElement->ongoing = 0;
$element->WorkElement->ongoingStartDateTime = null;

$element->WorkElement->save();