[Lonesome in Massachussets]

In the cases where it's been used, a criminal left DNA at the scene of a crime (generally on a corpse) years ago. When the DNA is compared to commercial voluntary databases, a relative of the perpetrator is discovered, and a search of the matching individuals relatives turns up the suspect.

I met a second cousin this summer, and he asked me about a DNA match with the same last name as mine. Turns out it was my nephew. He would never have know without this tool. Also, people have found siblings whom they never met through these tools. On balance I think it's a good thing. One drawback is when women put a child for adoption, the child might use a relative's DNA to find her. More women may be reluctant to give up children for adoption.

[Little Ray]

Just because these databases have been used appropriately so far does not mean they will used properly in the future.
Just imagine what the National Socialists could have done with such a database, and you start approaching the worst case scenario.