If the Allow Default Form setting is enabled then the default form(s) you've set up would show in addition to any other subscribe form that you have manually placed on a page.
Please note, that the Allow Default Form setting is only available on the Manual Placement form type.
If enabled, then two subscribe form will show on the page. You'll only want to enable this setting if there are multiple widgets being placed on a page.
How to Enable
Step 1: Navigate to Grow Publisher Portal > Subscribe > Manual Placement Form > Advanced. Simply toggle on the Allow Default Form setting on if you want to have your default form(s) display on the same page as the manually placed form.