This is an important question. Classroom management is important for all ages and this includes toddlers and elementary school aged children. There are several reasons for this.
All children need to learn boundaries. It seems that one of the innate tendencies in children is to test boundaries. They want to go as far as they can go. This can be very detrimental to the whole class. So, by managing a class well, you set the limits for children. This is also healthy for them.
Second, a well-managed class sets a good example for children as they move on. If they can learn good classroom behavior early on, it will go a long way towards their stance towards education.
Finally, by managing a class well, you enable other children to learn as well.