I got e-mail about it recently:

To make it easier for people to reach you over video call or text, the phone number(s) for your DEVICE_NAME are being verified and added to your Google Accounts.

How it works

Google will periodically obtain your number from your carrier by exchanging device info, such as your SIM card identifier, to enable Google services (standard message and data rates may apply).

You can manage this setting anytime on your DEVICE_NAME by going to Settings > Google services > All services > Phone number verification.

Verified numbers are added to all Google Accounts signed in to your DEVICE_NAME. To manage the phone numbers in your account, go to account settings.