Subform Best Practices: Blank vs Existing Form

Subform Best Practices: Blank vs Existing Form

I would like anyone's input as to what the BEST PRACTICES are when adding a Subform to a form in Zoho Creator. The two options given are:
  1. Blank Form: This is when you can add the fields directly to the subform.
  2. Existing Form: This is when you can connect it to an existing form in your application.

What I don't understand is why you should choose one over another. Is there a downside to one or the other?

Example:
What started me down this path is I generally create a blank subform when I am trying to add "items" to an order. But then I installed the "Laundry Management" app from the App Deck. And in this particular case, when a user creates a new order, they used an Existing Form, "Order Details" when adding a subform. Why did they do this? Is it better for reporting? What am I missing here?

My Specific Scenario:
In my app, I have an order form, and I have "items" as the subform. Lots of lookup fields in this blank subform. Why would I create a separate table to write the "items" into instead of just having as a blank "subform"?

I just want to make sure that I am building my app with the best practices in mind.

Any thoughts would be a great help. Thanks!