Problem:
I dont know when or why I should or shouldnt use sweep intervals

Solution:
The use of a database defined sweep interval really depends on what your 
application does and what type of maintenance you want to employ on your 
database.

As a general rule, use what works best for your database needs.

  If your application uses very quick transactions you may not need to use it.
  If your application has both long and short running transactions you most 
  likely will want to employ some type of db sweeping.

  Choosing a sweep interval is totally application and performance dependent.

  IMPORTANT:
  If you disable the sweep interval, make sure that you either manually 
  sweep the db periodically or as needed (when performance is suffering) 
  or run scheduled gbak and restores without the -g switch used.