SSI and Child Support?

I have a friend that draws SSI and he got a divorce. In my state, Tennessee, there is a primary parent and secondary parent, determined by the amount of time that each parent has the child. The secondary parent is who pays child support because the child lives with the primary parent also the amount paid is determined by this.

Now here is my question; why is it that the secondary parent receives the SSI, also receives the child's benefit, and then he pays her the amount of child support that was ordered by the court? I also know that the child's benefit is about four hundred dollars more than what the court makes him pay.

By BK from TN