I'll provide a simple Server Side Real World Example here, say if the records are looped and each record has a form with a delete button and you need to delete a specific record, so here comes the hidden field in action, else you won't get the reference of the record to be deleted in this case, it will be id
For example
<?php
if(isset($_POST['delete_action'])) {
mysqli_query($connection, "DELETE FROM table_name
WHERE record_id = ".$_POST['row_to_be_deleted']);
//Here is where hidden field value is used
}
while(condition) {
?>
<span><?php echo 'Looped Record Name'; ?>
<form method="post">
<input type="hidden" name="row_to_be_deleted" value="<?php echo $record_id; ?>" />
<input type="submit" name="delete_action" />
</form>
<?php
}
?>