Partner function to account group

Before posting this query searched across the forum, but did not get a fulfilled answer. My query is In Partner determination procedure we do a assignment of partner function with account group, please let me know the clear requirement for this assignment.

The basis for this question is, already my partner procedure is assigned to partner function and in turn partner procedure is assigned to account grp, this way there is a connection already built between partner function and a/c grp.

If this is the case why we need to assign partner function to account group??

Raj