Again, I appreciate the sincerity of your question, and I share your sentiment.
For example, let's say a party leader has access to the secret report of the National Security and Intelligence Committee of Parliamentarians, the version that could not be made public. He can then use the information contained in that report to make decisions related to his responsibilities as party leader.
You're absolutely right that a political party leader is always required not to disclose information, as are we here, and as are my agency colleagues.
That said, the information consulted by the party leader can still be used to make the necessary decisions on candidacies, for example.