The first and most important reason that classroom rules are imperative and must be reinforced is for the safety of the children in the classroom. Teachers must be alert at all times, not for just what is going on in the classroom, but also for what could happen.
The rules also teach the children discipline and self-control. Rules and structure are also important in the classroom because they will eliminate stress. If the students realize what is expected of them and those expectations are consistent and fair, the classroom will be a more pleasant place to be and the students will be more effective and engaged in their learning.
One thing that many teachers do is set up rules for their students, but then forget they they need to have a set of rules also. It is important for a teacher to develop a consistent, fair and inviting environment for their students.
Some good resource links are listed below.