Although it is possible to build relatively complex Drupal 8 sites without ever having to construct a custom module, it is likely that you’ll need to create at least one custom module to fulfill functional or technical requirements of your organization as the complexity of your solutions increase. Examples of when you may need to step into the custom module arena include these scenarios:
An off-the-shelf module meets most of your requirements but you need to augment one or more aspects of that module to address the unique requirements of your organization.
You need to modify how ...
