It's simply so you don't put too much milk in. Whether rationing or not, without eyeballing if the white liquid you put in the white cup is enough, or you need

It's simply so you don't put too much milk in. Whether rationing or not, without eyeballing if the white liquid you put in the white cup is enough, or you need 1ml more. If you put the tea in first and milk last, you see the colour change as you carefully titrate the milk in and disaster can be averted :)