As a teacher, it is helpful to know if there is something going on at home that may be affecting my student's performance in my class. For example, if parents are going through a divorce their children are not going to be able to focus in school.
Of course, if a child is being abused at home and I become aware of it, I need to intervene on the part of the child.
Some children feel that they are never "good enough" for their parents, and there are times when a teacher can communicate a concern like that to their parents. I guess my answer is that my primary concern is my student's welfare and academic progress. If knowing the family can help me be a better teacher for their child, then I want to know.
A teacher needs to know basic family information. Who is the child’s primary caregiver? Are both mom and dad actively involved in raising the child? Are there siblings in the home? How many, and are they older or younger? Are grandparents actively involved in the child’s life? Are there extenuating family circumstances, such as a parent away from home serving in the military? A teacher should also know about dramatic family changes. This could range from the death of a beloved pet, a divorce, or a seriously ill family member. Most importantly, a parent should always share with the teacher any information they truly feel will impact a child’s academic performance.