Aristotle has just finished his discussion of the different kinds of deductive argument. Now that we know what dialectic deduction is, he will present various reasons for learning it. Here is the second chapter of Topics, Book 1:
Next in order after the foregoing, we must say for how many and for what purposes the treatise is useful. They are three intellectual training, casual encounters, and the philosophical sciences. That it is useful as a training is obvious on the face of it. The possession of a plan of inquiry will enable us more easily to argue about the subject proposed. For purposes of casual encounters, it is useful because when we have counted up the opinions held by most people, we shall meet them on the ground not of other peoples convictions but of their own, shifting the ground of any argument that they appear to us to state unsoundly. For the study of the philosophical sciences it is useful, because the ability to puzzle on both sides of a subject will make us detect more easily the truth and error about the several points that arise. It has a further use in relation to the principles used in the several sciences. For it is impossible to discuss them at all from the principles proper to the particular science in hand, seeing that the principles are primitive in relation to everything else: it is through reputable opinions about them that these have to be discussed, and this task belongs properly, or most appropriately, to dialectic- for dialectic is a process of criticism wherein lies the path to the principles of all inquiries.
There are three reasons to learn dialectic: training your mind, average arguments and in philosophy. It is useful for training your mind because this book tells you how to train by giving a plan for training. It is useful in average arguments because we will be able to understand their argument and show where it fails rather than relying on our own opinions. Dialectic is also useful for philosophy. Dialectic will help us by first taking one side of a philosophical position and then the other. Doing so will allow us to understand the issue, notice falsehoods and become aware of the truth. Finally, there is a fourth reason to study dialectic. Any discipline such as science, history or carpentry has some basic principles that are assumed to be true by anyone who participates in the discipline. So no scientist can defend the basic principles of science as a scientist, for example. However, common opinions about these principles can be examined by dialectical reasoning. This same process can also help discover these principles by a process of criticism.
Aristotle is not very clear here on how exactly dialectic will help us discover the basic principles of a discipline. It is also not clear whether these principles are to be examined in general, or he is talking about how we discover them. I believe that he means both.
Next, Aristotle will describe his goal for this particular book.