The answer is: It adds credibility to the argument they wish to prove.
When conducting research, it is very important to learn how to rely on previous research in order to add depth, credibility and foundations for one´s own research. Science and discourse are built upon historical contributions by previous thinkers of all sorts, thinkers who, like any current researcher, put their arguments to the test, verifying them and seeking corroboration and communication within any given field of knowledge. Since science and discourse are intersubjective and historically built, one needs to cement one´s own original contributions, insights, and arguments on the contributions of others, from the past but also, evidently, from the present.
