Google asks for these security precautions to authenticate account ownership and to recover the account if necessary. Although you can omit phone number verification while opening an account, you will be asked for other details such as your birthday and gender.

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Phone number verification is a security precaution:

Google sends the verification codes to your phone number for account recovery and 2-Step Verification.
Recovery email is yet another security feature:

In the event of loss of access to your phone, you can use a recovery email address to confirm your identity and recover your password.

Google account recovery:

If you lose access to your account, you can use the account recovery procedure to recover, but you'll have to be able to verify ownership, which normally means supplying information relating to your phone or recovery email.

Skipping the phone number:

Although you are not required to enter a phone number when signing up for a new account, Google might request it at a later point in time, particularly if you are attempting to regain your account.