Salim - the idea is that the topic starter (who normally is the person who has creatde the mod), updates his (her) post so that you the user does not need to trawl through many posts to find the most recent version.
In fact the follow up posts probably make little difference to the mod in most cases, as any problems drawn to the attention of the author will have (hopefully) been corrected.
If the origianl poster deletes his post, then all posts following it will also vanish, so please do not worry too much
at the moment - but expanding all the time....) 
