- pay stays the same if WFH is from same city
- several employers other than Google have policies to adjust pay based on location
- the calculator they developed is to help make employees make data-based decisions (this is in contrast to other companies where no such calculator exists and one is left to guess and/or initiate talks with their boss that may or may not end awkwardly)
Another unspoken thing: there's such thing as salary negotiations and exceptions. "If I take a 10% cut, I'm leaving" is a perfectly reasonable thing to say in such a negotiation, and maintaining current salaries is something these types of employers are not culturally against (the way some middle management thinks of performance notwithstanding).
The general goal of the policy is to make things "fair" in the sense that they already have employees in lower-pay locations and it's unfair/potentially problematic to have two people in that same location doing similar work but being paid vastly different amounts. The "why don't they just pay everyone SV salaries everywhere" argument is obviously from people who never ran businesses and are just disguisedly wishing they made more money: saving money on salaries is done for the same reason companies do tax-related shenanigans. In Google's case, even the employees are kinda in on the scheme, as bottom line affects quarterly reports, and thus their equity compensation...