It allows the show to carefully plan a clear path for its storyline, which should minimize getting off track and prevent a forced abrupt conclusion (in the event of a tragic, unexpected cancellation).
Well, you can now add another series to this list: Nip/Tuck. FOX/tv-genres/fx has shared their plans for the remainder of this series and they are as follows:
The 5th season - abbreviated by the strike - will conclude in 2009 with 8 more episodes.
Then - in 2011 - a final 6th season consisting of 19 episodes will air (2011?! I just hope Julian McMahon isn't bald by then...).
This means that the show will conclude having filmed exactly 100 episodes.
Source: TV Guide