I think the Transcendental Doctrine of Method ('TDM') fulfils two roles : (1) it introduces or reinforces certain contrasts Kant wants to make between philosophy and other subjects and to differentiate his approach to philosophy from a number of other approaches; and (2) it provides an anticipatory link with Kant's ideas about practical reason and ethics.
TDM's contrastive role
TDM has four chapters (excuse me for stating the obvious) :
The Discipline sets out Kant's most developed views on the relation of philosophy to mathematics, stressing that both admit of synthetic a priori cognition but that mathematics can solve its problems definitively while philosophy can only define general principles and the conditions for the possibility of experience of objects. Mathematics can be determinate to a greater degree than is possible in philosophy.
The chapter then wanders off a bit into an argument for the freedom of public communication. I can't see that this really belongs.
Leave the second chapter for a minute. The third chapter, Architectonic of Pure Reason, continues the distinction between philosophy and other subjects, e.g. historical knowledge, and specifies in more detail the difference(s) between theoretical and practical reason.
The fourth chapter, History of Pure Reason, provides a high-level contrast between the transcendental philosophy of the Critique and dogmatism, empiricism, scepticism and indifferentism.
So chapters 1,3 and 4 serve to differentiate philosophy more thoroughly from other subjects and to mark off the Critical appproach from dogmatism, empiricism and the rest. Their principal role is contrastive.
TDM's anticipatory role
The second chapter, to return to that, the Canon of Pure Reason, makes the anticipatory link with practical reason and ethics. It 'contrasts the epistemological status of theoretical cognition with that of the principles and presuppositions of practical reason or morality, and in so doing provides Kant's most systematic discussion of moral philosophy prior to the Groundwork of the Metaphysics of Morals (1785) and Kant's first systematic statement of his argument for rational faith in God on moral grounds' (Guyer & Wood Introduction to I. Kant, Critique of Pure Reason, Cambridge : CUP, 1998, 19.)
It is hard to see what the argument for rational faith in God on moral grounds is doing here - to work out why Kant includes it - but the contrast between the epistemological status of theoretical cognition and that of the principles and presuppositions of practical reason fits with the general contrastive concerns of TDM. It also underlines that Kant has unfinished work to do on reason; he has to cover its practical side; it prepares us for the Groundwork and the Critique of Practical Reason as integral to the task of which the Critique of Pure Reason is only the first part and major component.
Guyer & Wood Introduction to I. Kant, Critique of Pure Reason, Cambridge : CUP, 1998.