Some disbelievers of Hadith say why Allah has not put everything in Quran; why Hadith was necessary as a wahi parallel to Quran? Actually there is difference of nature between Quran and Hadith; Quran tells us what we should do and what we should avoid, but Hadith tell us how people can practice/ act upon what they are supposed to do and what they are supposed to avoid according to Quran. You know when people start acting upon a scripture there may be thousands of variations/ differences as to how scripture should be acted upon. Hadith is a MODEL of ways and means we must go for to act upon Quran in all times to come. This Model is based on people's acts/omissions to follow Quran as well as our Nabi's acts/omissions/sayings; the people acted/ omitted to act in their lives, and our Nabi ﷺ declared such acts/omissions as right or wrong in the light of Quran and wahi ghair-matlu. Suppose if a Model based on people's acts/omissions regarding following Quran was to be inducted in Quran, in that case- for people considering only Quran as a revealed guidance- all people's acts/omissions to be committed till Qiyamah should be explicitly described in Quran as part of such Model- and obviously such a Model is not in line with Allah's scheme of Things , which is based on Trial, and knowing every thing already is repugnant to scheme of Trial. The other option was to rely on such a Model in which acts/omissions of people living in our Nabi's s.a.w.w. age were described and principles described in such Model were to apply in all events by all people to come tillQiyamah; and Hadith is a form of such Model.