The sunnah is the reality or the actual statement, act or tacit approval of the Prophet صلى الله عليه وسلم - what he actually did, said or approved of. There is no such thing as a "weak sunnah" or a "rejected sunnah." However, that actual sunnah is captured in the reports that have come down from the Prophet صلى الله عليه وسلم, which constitute the hadeeth literature. As noted before, not every report is correct and substantiated - indeed, some reports are even blatant fabrications. In other words, the entire hadeeth literature does not represent the sunnah of the Prophet صلى الله عليه وسلم. Only the acceptable ahaadeeth represent and portray the real sunnah of the Messenger of Allaah صلى الله عليه وسلم.
Shaykh Ibn Taymiyyah رحمه الله wrote, for example, "The sunnah that is a must to be followed, for which one is praised upon following it and blamed for going against it, is the sunnah of the Messenger of Allaah صلى الله عليه وسلم in matters of beliefs, matters of worship and the rest of the affairs of the religion. And that is known only by knowledge of the ahaadeeth of the Prophet صلى الله عليه وسلم that are confirmed on his authority." 
Once the role and the importance of the Sunnah in Islaam is made clear, the importance of the ahaadeeth should be very obvious. One needs to know what the sunnah of the Prophet صلى الله عليه وسلم was. One also needs to know where to look to find the statements of the Prophet صلى الله عليه وسلم. By the grace of Allaah سبحانه وتعالى, unlike the Messengers عليهم الصلاة والسلام who preceded the Prophet Muhammad صلى الله عليه وسلم, Allaah سبحانه وتعالى has preserved for Muslims the actual statements and actions of His last Prophet صلى الله عليه وسلم. The Prophet's صلى الله عليه وسلم statements, actions and even his physical appearance are all captured in the hadeeth literature. Virtually nothing has been lost from his noble life. A Muslim can know exactly how he prayed, fasted and participated in daily matters with his fellow companions رضي الله عنهم or with his wives رضى الله عنهنّ. Such a miraculous and complete record cannot be found for any other historical figure since the creation of man. And this is actually another sign of the truth of his Prophethood, and of the fact that he is the final prophet, after whom no prophet is needed. 
 Majmoo' Fataawa Shaykhul Islaam Ibn Taymiyyah (Riyadh: Daar al-Iftaa, n.d.), vol. 3, p. 387.
 The Authority and Importance of the Sunnah, pp. 29-31