What is the purpose of Stem cells preservation ?
Future medical breakthrough are going to come through stem cells. These cells have the ability to convert into anything like the kidneys, heart, lungs etc. Also, many diseases like cancer, diabetes etc can also be treated.
But, for now it's an on going research. You can think of it as a insurance policy against unforseen future happenings. But, for problem with this technology is that only the parents or siblings along with the baby can make use of these stem cells. So, every person would have to preserve there own stem cells for it to work.
And the best source of stem cells is the ambilical cord when the baby is born.