Groups can be useful to distinguish active from inactive learners, separate learners by enrollment key used, or role/position/title across all departments. However, I haven't found any pages that allow filtering by group, which kind of defeats the purpose.
For example; my users do not have access to remove learners from the database, instead they place those learners which are ready to be removed in a group so that they can be identified and removed by the System Admin. Course completion data must be collected on these learners (which reach across multiple departments and enrollments) before removing them, so the Learner Progress report is run for these learners.
Because the reports and users cannot be filtered by Group (but can be filtered by Notes), a Note must be added to each learner in order to filter properly.