Why is there both a “Save & Apply” and a “Save” button in LuCi?

Issue: When changing configurations in LuCI, I see a “Save & Apply” button and a “Save” button. Why two buttons and what is the difference between them?

Solution: In LuCI, you can do several different changes in different tabs, each time clicking “Save” without committing the changes. You can then use “Save & Apply” to commit all of those changes in one transaction.