Benefits of sub partitioning by hash

Oracle Version 10g2  

We have a typical sales items type table that is partitioned by date and has local bitmap indexes on product code and site code.  

I was wondering what are the benefits of sub partitioned by hash on either the customer number or the order number?  

Typical queries just sum up quantities sales over a give date range. However we do have some queries which will self-join the table i.e. For a given order what other products are in that order  

We do use parallel hints in the queries as well  

Any insight will be much appreciated  

Rob

--
http://www.freelists.org/webpage/oracle-l