docs: clarifying why site and module projects exist

oeps/architectural-decisions/oep-0065/decisions/0003-frontend-projects.rst

@@ -43,6 +43,8 @@ There are two primary types of projects: *site projects* and *module projects*.
 Site Projects
 =============
 
+Site projects serve the shell, contain the bulk of the frontend's configuration, and are a place to check in customizations and extensions to the shell and modules built as part of the project.
+
 A site project uses a :term:`Site Config` file to define the configuration for a :term:`Site`, which is an independently deployed portion of the Open edX frontend which renders a header, footer, and loads one or more Modules.
 
 A site project consists of:
@@ -67,6 +69,8 @@ To describe the steps in the above image:
 Module Projects
 ===============
 
+Module projects are a place to put customizations and extensions to federated modules, and contain configuration specific to those modules.
+
 .. image:: ../module-project-architecture.png
 
 A Module Project uses a ``config`` data structure in ``package.json`` to define what modules it should bundle for :term:`module federation`. This mirrors the ``config`` data structure in the :term:`module libraries <Module Library>` (``frontend-app-*`` repositories). It also uses a :term:`Module Config` file for additional configuration of those modules beyond that which will be supplied by the site's :term:`Site Config`, i.e., module-specific configuration.