What is the importance of prefaces written by Aristotle to his books?

It was a habit of Aristotle to write prefaces to his books. In these books he invariably noted all achievements made up to that date on that subject by others before him. It has been alleged that he did this only to disprove and discredit each. Francis Bacon once said that Aristotle could not reign secure without putting all his brethren to death. However, thanks to these prefaces, the world now knows what thought and knowledge had been there before the time of Aristotle, Plato and Socrates.