Add new content location refers to any location on your site that you want to manage content for. In most cases this will be an existing page of your site. This could also be a shared location, such as a side navigation that is the same on a number of pages (see the Shared Elements content in the PowerCMS demo).
Add new page from template is only available when you have chosen to make a content location a template. If you select the green + icon that displays when hovering over the template name, you can create a new content location that contains all the same content areas as the template. It is these pages (the ones based on a template) that do not require the Developer Notes for, because you will be provided a URL variable that can be added to the end of the template's URL.
For a Super Admin, these two options may seem very similar, but for the additional admin accounts (your clients) the only option they will have for creating new pages on their site, is the Add new page from template option. This allows you (the developer) to define what your client can create but also giving your client some more control of their website.