If the account is deleted, all posts will also be deleted or sometimes the username of OP will change to something else to keep the posts. It always is better to keep the posts because some of them maybe important.
Edit: shorena already answered.
it should be possible keeping posts yet deleting accounts. only thing, the username on the post seems anonymous or something like that. I think that should be a right for users to be able to do that.
people generally want to go away with their content.