What's the point in waiting 6 or 12 months before allowing researchers to make their Belgium-funded article open access? Belgium recently passed a law allowing researchers to make any of their article open access if the majority of the research funding  came from Belgium public funding

Here is the law:


  Art. 29. Dans l’article XI.196 du même Code, inséré par la loi du
  19 avril 2014, il est inséré un paragraphe 2/1 rédigé comme suit:
  
  “§ 2/1. L’auteur d’un article scientifique issu d’une recherche
  financée pour au moins la moitié par des fonds publics conserve, même
  si, conformément à l’article XI.167, il a cédé ses droits à un éditeur d’un
  périodique ou les a placés sous une licence simple ou exclusive, le droit
  de mettre le manuscrit gratuitement à la disposition du public en libre
  accès après un délai de douze mois pour les sciences humaines et
  sociales et six mois pour les autres sciences, après la première
  publication, dans un périodique, moyennant mention de la source de la
  première publication.
  
  Le contrat d’édition peut prévoir un délai plus court que celui fixé à
  l’alinéa 1er.
  
  Le Roi peut prolonger le délai fixé à l’alinéa 1er.
  
  Il ne peut être renoncé au droit prévu à l’alinéa 1er. Ce droit est
  impératif et est d’application nonobstant le droit choisi par les parties
  dès lors qu’un point de rattachement est localisé en Belgique. Il
  s’applique également aux œuvres créées avant l’entrée en vigueur de ce
  paragraphe et non tombées dans le domaine public à ce moment.”.


Google translate:


  "§ 2/1. The author of a scientific article from a research
  financed for at least half by public funds, even
  if, in accordance with Article XI.167, he has assigned his rights to a publisher of a
  periodical or has placed them under a single or exclusive license, the right
  to make the manuscript freely available to the public for free
  access after twelve months for the humanities and social sciences
  and six months for other sciences, after the first
  publication in a periodical, subject to mentioning the source of the
  first publication.
  
  The publishing contract may provide for a shorter period than that set
  paragraph 1.
  
  The King may extend the period laid down in paragraph 1.
  
  The right provided for in paragraph 1 can not be waived. This right is
  imperative and applies notwithstanding the law chosen by the parties
  since a point of attachment is located in Belgium. he
  also applies to works created before the entry into force of this
  paragraph and not fallen into the public domain at that time. "


What's the point in waiting 6 or 12 months before allowing researchers to make their Belgium-funded article open access?
I think you are reading this wrong. It's not that you are not allowed to make something open access before this time span, it's that you have to after it when public money has been used. This is called Delayed Open Access (thanks to Anyon, who mentions this in a comment).

Specifically note that the journal is free to open up the paper earlier:


  The publishing contract may provide for a shorter period than that set paragraph 1.


(for instance, if you paid the open access fee). I think the most salient point is this:


  The right provided for in paragraph 1 can not be waived.


That is, you cannot sign a copyright transfer to a closed-access journal that gives them the right to not make it open access even after the closing period is over.

If you are asking why they don't require immediate open access, you are back at Najib's answer - publishing is a business, and presumably the Belgian legislation has decided that Delayed Open Access is an acceptable compromise between the publisher's interests and the public's interest in free access to scientific information.