Hi Ludwig,
Let's say that for example I want to attach a PDF file (with some comments, details about the vendor and the purchase order) to my purchase order using the paperclip icon.
Thus, I do it before confirmation. Lets say that this action was undertaken by the purchasing agent.
Then, it is time for invoicing and this screen is only available for the accounting people. Before generating/ posting the invoice they also have the opportunity to add some attachments 
The problem here is that they don't see the attachments that were uploaded by the purchasing agent beforehand (and linked to the purchase order). And it could be problematic if this information was important to share with the accounting department.
In conclusion, the attachments are not following from the PO to the invoicing process. Yes you can see it afterwards in the invoice journal, or vendor transaction but then it is too late since the PO invoice was already posted.
Best
Unai