The physicist Richard Feynman used to tell the following story:

(...) There have been many experiments running rats through all kinds of mazes, and so on - with little clear result. But in 1937 a man named Young did a very interesting one. He had a long corridor with doors all along one side where the rats came in, and doors along the other side where the food was. He wanted to see if he could train rats to go to the third door down from wherever he started them off. No. The rats went immediately to the door where the food had been the time before.

The question was, how did the rats know, because the corridor was so beautifully built and so uniform, that this was the same door as before? Obviously there was something about the door that was different from the other doors. So he painted the doors very carefully, arranging the textures on the faces of the doors exactly the same. Still the rats could tell. Then he thought maybe they were smelling the food, so he used chemicals to change the smell after each run. Still the rats could tell. Then he realized the rats might be able to tell by seeing the lights and the arrangement in the laboratory like any commonsense person. So he covered the corridor, and still the rats could tell.

He finally found that they could tell by the way the floor sounded when they ran over it. And he could only fix that by putting his corridor in sand. So he covered one after another of all possible clues and finally was able to fool the rats so that they had to learn to go to the third door. If he relaxed any of his conditions, the rats could tell.¹

Feynman used the story to make a point about the way science should be conducted, but the story could also illustrate why we need to use minimal pairs to learn Chinese tones. In this context, as students of Chinese, we are the rats. If your teacher says something that sounds like "ni shen me shi hou qi chuang?" you might realise quickly that you are being asked what time you get up but not be able to say with absolute certainty that you recognised the tones. Guessing from context is akin to the rats in the experiment finding the door from the extra clues they could pick up. With minimal pairs all the extra clues are eliminated.

The same thing applies to language production. Your teacher understands you, but that doesn't guarantee that you actually got your tones right. Teachers are used to hearing students and working out what we are trying to say. Since they are expecting a certain answer they may go some way to convincing themselves it is what they heard - or sometimes they might just be too polite to keep correcting us. But when your teacher has to press a screen button to indicate what tone you used then you have a measurable, gradable way of testing yourself and plotting your progress.

1. Richard P. Feynman Cargo Cult Science California Institute of Technology 1974 commencement address.