And as yet another example, PS is very good at indicating the passage of time in the composition of a message. As in--

Dear Joe,

I think that the databases are hosed, the webservers are down, and the customer service reps have just gone on strike.


ps: just got a phone call from tim, webservers aren't all the way down.

pps: before i hit send tim called back. the webservers are hosed.

Before you say that the ps's should be dropped, consider that they provide other information than just the words. For one thing, they communicate that the people in the above situation are currently rushed and unsure of what is going on--something that would not have been communicated without them.