Document placeholders are perfect for documents that are required for a transaction, but aren't necessarily a template themselves. A good example of this is a scanned check or receipt. If you keep your templates organized into packets, you'll immediately recognize the benefits in the example below.
Here, we have a buying packet that I've created.
If we assume that I need a copy of the earnest money check, I obviously cannot make a template for that. However, what I can do is make a placeholder that reminds anyone using this packet they need this document. Clicking the arrow to the right of Add Document allows me to choose Add Document Placeholder.
After adding my new placeholder, I click the down arrow to the very far right of it's title, allowing me to choose RENAME. We can now rename the placeholder.
You now have a placeholder available to be pulled into any loop that includes this packet!