Labels are useful for a quick overview of the work list. With a glance, you can see why an invoice is still in the 'New invoices' category, for example. You can also use labels to indicate that you are still waiting for an attachment from a supplier. This allows you to classify/personalize your overview a lot better *.
Labels can be created in the option: 'Options' in the worklist (see image above).
You can select an icon yourself with a corresponding description and add it in the invoice view to an invoice.
* The labels assigned to invoices are also visible to other users and, depending on his/her rights, editable.