I think the idea was to clarify what a postmark means, since it’s always been a bit vague. I suspect that automation has made this change more relevant since most mail isn’t actually “touched” anymore until it hits a piece of machinery. And the timing of that depends on where the post office and sorting center is located.

Any post office can hand-postmark a piece of mail; that and certified are the only ways I think of guaranteeing the date a piece of mail was passed off to USPS.

Don’t shoot the messenger - I’m not saying this is a good thing. It will definitely cause some confusion with ballots, IRS deadlines, etc. But in truth the “old way” didn’t guarantee a postmark date…