Why is it important to do cvs or amniocentesis?

Personal decision. Both tests are 100% accurate in determining whether or not the baby has chromosomal problems like down's syndrome. Women over 40 have a 1 in 20 risk of this. Some people would terminate the pregnancy if they knew the baby had a chromosomal problem. The cvs gives you info earlier than the amnio but has a higher miscarriage rate.
If at increased risk. It is only indicated and may be considered in which there is an increased risk of genetic disorder. If you are 35 or older by the time your baby is due if you previously had a baby with down syndrome or another disoeder like spina bifida and you and your partner are known carriers of genetic disorder like cystic fibrosis.