A twin test determines whether the twins are monozygotic or dizygotic i.e. if they come from one fertilized egg (monozygotic) or whether they come from two fertilized eggs (dizygotic). The analysis is based on the assumption that monozygotic twins are genetically identical by sharing the exact genetic material, while dizygotic twins share only half of their genetic material.
This information could be useful in many cases:
1. Personal reasons; when the parents want to know if their twins are identical or not
2. Medical reasons: blood transfusions or organ transplant where it is important to know whether the twins are identical as monozygotic twins are the perfect donors for each other (best chance for organ acceptance).