All Muslims, be they Sunni or Shia, agree that hadiths are essential to understanding Islam. Hadith are important because without them the Qur'an does not make sense. They provide a context to the verses in the Qur'an. The Qur'an is a rather concise book and therefore contains many general statements. For instance, the Qur'an commands Muslims to pray, but it does not provide the details of how one is to perform prayer.
The Qur'an also commands Muslims to perform pilgrimage and give charity, but it does not provide the details. These details are found in the hadith where the Prophet peace be upon him explained the details and mechanics of prayer. There are dozens of verses in the Qur'an that command Muslims to follow the Prophet. Without knowing his teachings one cannot possibly fulfill this command.