Groups are used to segment your Learners for training purposes. You can manage and deliver different content to each Group. For example, you can have your staff in one Group and contractors in a separate one. You can now manage separate training content, access controls, and delivery options for each Group.
Please note, there is no need to create a separate Group for each department. If you are looking to segment your learners for reporting purposes, please see Adding/Removing Departments and Tags