We will often want to use custom or community modules with our Odoo instance. Keeping them in a separate directory makes it easier to install upgrades to Odoo or troubleshoot issues from our custom modules. We just have to add that directory to the addons path, and they will be available in our instance, just like the core modules are.

It is possible to think about this module directory as an Odoo environment. The Odoo start command makes it easy to organize Odoo instances as directories, each with its own modules.