They make it easy to have independent administrators for separate filesystem areas - without any access for the other administrators. The ACl inheritance scheme with masks and the accumulation of rights from user, several group and role entries are sure difficult to emulate with roles and types - you would need a real lot of them. Again, one important focus lies on separation of duties. With ACL model, every user can maintain her own set of user groups, which can optionally be used by herself or other users for administration. E.g., one user maintains all user groups, and another grants rights for these groups.
None can work alone.
To avoid social problems for workgroup members, only the workgroup leader knows who is in the group. The workgroup has its own filesystem area on the company server. Workgroup leader L defines a private user group. Now L can add or remove users to or from the private group, without anybody else's knowledge. L cannot grant individual access to objects.