sid/serial# vs. audsid: why both?

>From the Fine Manual:

Oracle9i Database Performance Tuning Guide and Reference, Release 2 (9.2) Part Number A96533-02
Chapter 24 Dynamic Performance Views for Tuning http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96533/sqlviews.htm#32598 V$SESSION

. SID: Session identifier, used to join to other columns 
. SERIAL#: Counter, which is incremented each time a SID is reused by another session (when a session ends and another session starts and uses the same SID) 
. AUDSID: Auditing session ID uniquely identifies a session over the life of a database. It is also useful when finding the parallel query slaves for a query coordinator (during the PQ execution they have the same AUDSID)

Oracle9i Database Reference Release 2 (9.2) Part Number A96536-02
V$SESSION
http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96536/ch3171.htm SERIAL# ... Session serial number. Used to identify uniquely a session's objects. Guarantees that session-level commands are applied to the correct session objects if the session ends and another session begins with the same session ID.  

In what cases does the SERIAL# need to be used? Can someone give an example where a session-level command would be applied to an incorrect session object if SERIAL# were not available?

Why not use AUDSID all the time? Is there a reason why the database keeps track of two session identifying numbers?

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