Used on internet forums that have a 'Show new replies to your posts' feature. Simple Machines forums have an 'Updated Topics' page, where a user can see topics they have posted in become updated as new replies are posted.
A user may post in a thread simply to receive new replies in the 'Updated Topics' page. However, if someone just posts "PTW", it could be seen as unconstructive and spammy.
user2 - Cool. PTW.