The Sunnah is the way of life Muhammad live and the things he did. The Function of it provides a model for Muslims and the ways they should live. It almost like steps on how to live life like a Muslim or Muhammad in this case. It like the way of life there expected to be living. For example rules , we have rules on how to act in school. So the Sunnah is like rules to live life like a Muslims and how to live it properly or at least accurate.