Its mainly for those organizations that have a separation of duties, where

- one group is responsible for installation/management of database software
- a different group is the dba's.

or it might even be one group owns "all" of the oracle software (database, EM, agents, weblogic, OID etc etc etc), and only a portion of that software should be accessible to dba's.

It's not uncommon for places to set 'dba' as the primary group and use that exclusively if it reflects the organization structure of a business (ie, "dba has to do everything")