Is a post that is not supporting or in contrary to the poster idea a shit post? I really want to understand this concept.
Depends what you mean by "supporting". If you mean having the opposite opinion of the original poster, then that is fine. In fact, that's usually the way things go around here, someone makes a point, and others usually try to correct them on what they believe was wrong.
If you mean not on topic, as in not contributing to the original posters idea, and your replying to another user in the thread then it could be considered off topic depending on the context.
Posts are been deleted when it's not related to Bitcoin or Bitcointalk forum. No moderator will delete a post that will benefit members of the forum
Not totally accurate the way you say it, but moderators will only delete posts which they believe break the guidelines. That is up to their discretion. I'm sure there has been times where a post technically might have been useful to certain users, but it wasn't on topic in a thread. For example, if you told me how to run my own node, in a thread about Bitcoin exchanges, then that would be off topic, despite potentially being useful to other users.