Thats the corresponding function from the helper.php
case 'alias':
// If this is an alias use the item id stored in the parameters to make the link.
$item->flink = 'index.php?Itemid=' . $item->params->get('aliasoptions');
break;
So if you look at the function in default.php you will see this piece of code:
foreach ($list as $i => &$item) :
$class = 'item-'.$item->id;
and following with an if clause and after that the code you are asking for.
So what it basically does (in my understanding):
Use the item id defined in itemparameters (basically its just the item id xD ) if the link is just an alias for another menu item. Without it Joomla shouldnt be able to set the correct active menu links.
You can set Menu Item Aliases by choosing them in the Menu Item Type selection: "System Links -> Menu Item Alias", when creating or editing menu items ;)
I hope this helps ^^