Essentially, why he was so important is taught directly to you in the SFBH syllabus
The section 5 dotpoints under
"5. MacFarlane Burnet’s work in the middle of the twentieth century contributed to a better understanding of the immune response and the effectiveness of immunisation programs" in which you learn about B cells, T cells and antibodies are why MacFarlene Burnet's work was so important, because understanding the immune response enabled us to be able to develop vaccines and health measures for infectious disease as we understood the immune response. For example, we needed the knowledge to develop vaccines as we knew about the different response rates due to the presence of memory cells, and also the about antigens to know how to make attenuated/killed vaccines to stimulate the response but not actually make the patient sick.
MacFarclane was most well known for his work on an influenza vaccine. He also did work in mapping the mutation rates of virulents such as influenza. His studies into agglutination are very important for immune response as well.