Jem:
At one time anyone trying to submit a post of over 2000 characters would simply receive a message stating that their post was too big. Unless they realised that their post would reappear by simply clicking the 'Back' button (which many people didn't realise) that meant that they had to type it all out again.
More recently, anyone posting over 2000 characters simply had the end of their post cut off, without any warning. If people failed to check what was on the screen after they'd clicked 'Submit', that could mean that their post became meaningless because important elements of it (such as links) had been excised.
The new counter facility is there to let people know when they're getting close to the (new) 4000 character limit, so that they don't experience problems when trying to submit lengthy posts.
Chris