There's quite a big difference between the DNA you leave in a forensic sense all around you (which would take a lot of effort and expense for someone to gather

There's quite a big difference between the DNA you leave in a forensic sense all around you (which would take a lot of effort and expense for someone to gather against your will) versus serving it up on a plate in a format that is trivially searchable for close matches against any relatives. The value is in the fact that millions of people's DNA is gathered in the same place in the same format, not the individual value of your DNA. It's a classic network effect.