Mutsy - I took that test once. It was kind of fun. I also have to say that many private schools offer math assessment tests along with reading assessment tests in order to determine if the child has adequate knowledge to function in the school.
These schools cannot rely on report cards because the curriculum taught is not the same in all schools. A student might come from a school that offers grade level courses while another school might offer a more accelerated curriculum.
For example, my children go to a private school in which the math and most of the curriculum is taught at the next grade level. So if my daughter is in 4th grade she would actually be doing 5th grade work.
Many of the competitive schools are designed this way which is why they have to have the skills assessments tests in the first place.