I've never used pipes myself, but ... extracted from 
/user/oracle/rdbms/admin/dbmspipe.sql


  --------
  --  USES
  --
  --  The pipe functionality has several potential applications:

....

  --    o Alerters (non-transactional).  You can post another process
  --      without requiring the waiting process to poll.  If an "after-row"
  --      or "after-statement" trigger were to alert an application, then
  --      the application would treat this alert as an indication that
  --      the data probably changed.  The application would then go read
  --      the data to get the current value.  Since this is an "after" 
  --      trigger, the application would want to do a "select for update"
  --      to make sure it read the correct data.
....

This would be similar to alerts, but the applicatiom wouldn't need to block,
could presumably conceive of a single server which does periodic reads of
the frequently updated table, and checks on pipes now and then to receive
notification of updates on less frequently updated tables.