The Qur'an claims to be perfectly clear and a guidance for humanity. Yet, there are significant differences between different sects, scholars, and views on abrogation, jihad, and legal rulings. This and the Qur'an is not well understood without the hadith.
Either the Qur'an is not as clear as it claims (challenging its divine attribute of clarity), or its guidance is incomplete without external aids (undermining its sufficiency as a standalone revelation).