Confused !?

So I see a lot of people are getting the scans for Down syndrome , but what I don't understand is if they're going to do these scams why do they have us do the blood test as well for the state ? I have taken blood work twice at 13 weeks and at 18 weeks to see if my baby had Down syndrome and because the state of California requires it . Both test came back negative so why is the ultrasound required for that ?