because you're cattle. the point of your life is you work for them and you get shitty paper with no value in return, that they allow you to trade in for the cheap shitty plastic toys and food they sell. since your purpose in life is to consoom more shitty plastic toys and food, you will work harder to get more of that paper to trade in for more toys and food. this relies on you not being able to store too much of that paper, if you do you might not be motivated to work as hard as you don't need to bring in more paper that month. inflation ensures you lose money regardless if you consoom or not.  deflation does the opposite, now people can save up money and even worse they might even have time and money to plan for the future, which might make them harder to exploit as cheap labour.
Sweden