Before you can establish or sign in to your account, we may ask you to prove you're not a robot to help protect you from abuse. This additional phone confirmation helps prevent spammers from exploiting our systems. You'll need a mobile device to validate your account.
The major reason Google asks for your phone number is to protect your personal information. Google can authenticate your identity using your phone number. Once the account is linked to a real person, it is tough to hack since Google will notify you and issue you another verification code if someone tries to access your account.
Google needs your phone number to:
Verify you’re human— There are robots and automated programmes that search Google, which is why Google needs to verify that you aren't a spam bot. A Completely Automated Public Turing Test to Tell Computers and Humans Apart is also used to solve this problem (CAPTCHA)
Prevent identity theft—If you enable phone number verification, a hacker will be unable to steal your account. Your phone will receive an SMS with the code, and you'll be alerted that your Google account has been hacked.
Account recovery—If you enable phone number verification, you will be able to recover your account faster. You can also change your password much more quickly.
Read the multiple methods, Visit: How to Verify Google Identity