
For a Second Account
Google's official help center clearly states the fact that they limit the number of accounts each phone number can create. Once that ceiling has been reached, your sign-up attempt stops with "This phone number cannot be used for verification" and the fix Google suggests is to try a different number.

To Get Ahead of the Week
A newly added telephone number may need about a week, as stated by Google, before it can be verified and allow sensitive actions to be taken, like an account's password change. If in such cases you use a temporary free number, then by the time your Gmail password can be changed, it's more than likely that you won't have that number. With a private number, you will have the peace of mind that the inbox is yours even after a week or two.

For 1+ Separate Google Accounts
Each Google Account holds a recovery number and a 2-Step Verification number in two different fields, each set up on its own. Filling one with your own number leaves the other empty, so a rented line can take care of it and help enhance your account security.

To Avoid the Cleanup Later
Swapping out the number on your Gmail account will only affect some of the connected services. Others, such as Calendar, Chrome, Google Pay and your Business Profile, each hold their own copy of the linked number, and it is updated one at a time. A new account started on its own virtual number doesn't join that list of services that already have your history, financial information, or location details.

For Work and Personal Both
Sharing the same verification number between your personal and work accounts is not a safe practice. Also, in some cases Google asks for a number not connected to any other accounts. A second line means that the two are never joined by one number and this keeps one lost phone from locking you out of both.

To Keep Your Number Out of Ads
By default your Google account can be used to personalize the ads you see daily, and turning that off is a setting of its own. Google states it does not sell numbers to third parties, however it still uses them.