Policy Groups are helpful to Admins for proving a method to create a collection of different security policies.
Grouping a number of policies together, allows for more streamlined administration of your security matrix for your organization.
These groups, and how you define them is completely up to you - this can be by role, department, function, etc.
You can delete a policy group, however, if there are dependencies (linked policies, or users) you must first remove them.