Why am I asked to login into my 23andMe account when I register with "Your DNA family"?

Question:

Why am I asked to login into my 23andMe account when I register with "Your DNA family"?

Answer:

As part of our onboarding process, you need to prove that you are who you claim you are by logging into 23andMe.

Please note that your password for 23andMe isn't stored on our server but remains on your computer (for security reasons).

See also:

-> What data is downloaded by the "Your DNA family" app?