We entered in a genome era and personalized medicine! But for why do we need genetic tests and need to know our genetic make up? In present day, any medicine does not work for everybody for same diseases! But if we know why are we getting a particular diseases, we probably can make better medicine! Genes are behind most of the function in human body from developing eye color to muscle development to brain function to circulation of blood. Each human differs 0.1% less than each other for that we are so different. Thus if there is defects in genes, diseases can develop.
Genes are part of DNA and defects in genes means changes in its bases. Thus if we know base sequences in our genome, we may know the basis of contracting each diseases and predict which diseases we have possibilities to contracting. As we know our DNA sequence in our genome, we know possibilities of upcoming disease and can take prevention. We can plan our life better. Although it took over 10 years to sequence human genome with approximately $3 billion dollar, it is now possible to sequence anybodys' genome approximately in $1000.00.All you need to sacrifice a little bit saliva or blood and a little bit of money! But the enormous information can be retrived could be life changing!
As for example if somebody knows, he is susceptible to develop obesity or type 2 diabetes, can change lifestyle for not onsetting these diseases. If anybody has a high chance to develop breast or skin cancer, he can take prevention of not exposing the body in sunlight or operating breasts for not having the diseases. Any many more! Scientists are everyday working to relate variations in genes with the environmental factors for explaining a specific disease are developing with the interaction of the environmental factors, so we can control them.
Moreover, many cancers could be cured if diagnosed in early stages. Now DNA sequence based diagnostics for many cancers are rapidly developing. Also many drugs are in clinical trial based on DNA informations of the tumor. It suggests that if we know for which gene defect tumor appeared for a specific person or for which gene defect tumor is being malignant, a particular drug could work. Thus you need to sequence your DNA either from tumor biopsy or from blood for upcoming possibility of diseases.