When you say that a drug makes people feel better and you want to convey that information it is difficult with a function to say with x amount of drug the patient feels y amount better. But you can argue that there is a strong positive relationship between taking a drug and feeling better.
If the variables in an equation were reversed, the function is mirrrored about the line y=x, graph it and you will understand.
This is usefull when you want to change the relationship of cause and effect. Such as are people buying a certain stock because its price is going up or is the price going up because people ar buying the stock. you can switch x and y to see the different relationships graphically.