There's an incredible amount of uses for high quality genealogical data:

-Actuaries/insurance companies: Longevity is largely heritable

-Drug Development/Pharma: The newest drugs are all gene therapies and data sets provide amble grounds for deep neural networks. Identifying potential causes of curable conditions, or identify a marketable consumer base.

-Police/Surveillance: I believe a genealogy database is actually how the Golden State killer was caught.

Certainly there are other use cases, but these are the most obvious to me.

Is the first legal?

The second .. there are big public DNA databases, and they are adding more and more people every day

The third one seems the most important

Marketing based on genetic predispositions...

Because it's a powerful tool. You are your DNA, literally. And when a company has so many information about people, some other actor will want that information and becomes pricy.

Like instagram. Why did facebook paid 1 billion USD for a company that generated 0 revenue? For the amount of users and information they have.

And to twist a bit your question. Why is it so valuable AND sensible?

Because imagine in 10 years, you get a kid and he wants to get a job, and because you gave your DNA away to some random company, now owned by the biggest employer of your state, they do not hire him. The reason? He has 5% chance of developing a sickness which might drain the medical coverage of said employer.

We would be reduced to numbers and probabilities. Labels attached to our biology.

I am not sure DNA is that predictive...

Proven, documented genealogy, coupled with genetics, is the Holy Grail of medical and pharmaceutical research. It's worth billions, because it could hold the key to the cure for cancer and other diseases.