To Terminate?

So I had my first appointment. I heard the heart beat, saw the heart beat, and was told myvbaby was on track for 8 weeks 2 days. Then my doctor asked me if I wanted to take certain tests to determine if I wanted to screen my baby for Down syndrome or other possible genetic problems. I asked her why would it matter and she said that some people do it in order to decide if they wish to continue with the pregnancy. Why would anyone want to terminate? Has anyone declined these tests? Or am I the only one that was surprised with the question?