On a 4-carriage (one unit) train then it's a bit silly to play that, but on trains that split it's very useful. It's got nothing to do with seating.
Indeed, this is the reason for it, along with short platforms (not sure if this applies to Southern though). It would be better if it just announced the destination for that part of the train though.
I can't think of any regular ones though. A couple I've heard a few times are:
"This train is today in reverse formation. Standard class is at the front of the train. First class is at the front of the train."
"We are sorry to announce that the xxxx to yyyy has been cancelled. We are sorry for the delay to this service"
-or-
"We are sorry to announce that the xxxx to yyyy has been cancelled. This train is expected to arrive in 5 minutes"
"This is a platform alteration. The xxxx service to yyyy will now depart from [the platform it was going from anyway]"
Plus all the announcements on trains between Reading and Waterloo, where whoever was doing the voice was having a laugh...
*serious voice* "The next station stop will be Virginia Water"
*a couple of minutes later* "We are now approaching... !!Virginia Water!!!ZOMG LOLOLOL!!"