Because if you are changing 3000 posts with gsub and a complicated regex with a bunch of edge cases (fix: goo.gl, http://goo.gl, https://goo.gl, but don’t touch https://maps.app.goo.gl, or https://map.goo.gl, and maybe you get rate limited by goo.gl, and on and on) you might want to get back the post before you totally screwed it up!
It’s been very cool to be able to look at the edits and see the before and after and be able to revert! One version would make https://maps.app.goo.gl/abd12 into something like https://maps.app.https://maps.google.com/;lkajw3rpoazse;flknmase;faijserfasefklasdfa, for example.