When I lived in California I worked at a place that raised rats to sell to labs and schools. Let me tell you - the first day I walked in I was terrified!! All day, every day, all we did was handle rats. I worked in the "white rat" building, but other buildings had different types. And other than to be used for labs and schools for research, and (great answer Hank - the best reason!!) target practice, I have no idea why they are on this earth. However, by the time I was done working there, I could pick up a female rat and if she was pregnant I could determine within a day when she would be having babies. A very useful thing in "real" life.
Their rate of reproduction is horrifying too.