AB testing is how you isolate a change and measure the impact. It's the only real way to be able to associate cause and effect. Best you can do otherwise is measure something over time while making changes. You can try and correlate changes with outcomes but it's hard to be sure the change is what drove the outcome.
That sounds pretty accurate. Anyone who's gone through some econometric classes would know that split testing is (almost) the only way to get to (close to) causality - all the rest is black magic called correlation which can also lead you to the conclusion that Nicolas Cage's movies lead to people drowning:
