Some bacteria and moulds thrive even in a cold environment and they can contaminate food. Even if this affected food doesn’t make the people who eat it sick, the food will spoil much faster and go to waste. Given the price of food these days, that can represent a significant financial loss over a month.
In addition to these important health and financial considerations, dirty fridges have to work harder to maintain the desired temperature. As a result the lifetime of the fridge will be reduced.