What is the point of passing $id around?

This is a question for the code so far in Alena Holligan's tutorial, "Build a Basic PHP Website". I'm on the video called "Displaying Categories."

So I'm following how we're listing $catalog on the various pages, and accessing the $catalog data with our functions get_item_html() and array_category(), but I see this variable $id being used as well, and I'm trying to follow it.

The only place I see $id actually being utilized is in array_category(), when, with each iteration in the foreach loop, $id is added to the $output[] array. It looks like $output[] thus becomes an array of numbers, like 102, 103, 104, etc.

From that point on, $id is returned in the $output[] array, and is then used in catalog.php in this foreach function... foreach($categories as $id) { echo get_item_html($id,$catalog[$id]); } ... so I see that $id is used to identify which index of $catalog to use. BUT, in that above foreach function, when $id is passed back into get_item_html(), it's never actually used.

So is there any point to the variable $id? Couldn't the $output[] array from array_category() just be filled with $item variables instead of $id variables, and it would then be an array of array items?

Maybe this question isn't too important, but isn't using $id just kind of cluttering the code a bit, or is it intended to make it easier to follow? I just keep losing track of the variables... :)