It seems obvious that even though parents choose to stop living together, the child's financial needs do not cease.
Child support is society's way of organizing the financial contribution of both parents.
If parents can not agree on a plan, then a predetermined equation is used by the Court to establish an amount of money one parent gives the other for their child's well being. More families fight about child support than not. It is also true that the kids always lose in these arguments. Court battles based solely on money have ruinous consequences. I experience first hand how parents' money disputes effect children.