Your reader already understands the basic concepts of your theory. So you don’t have to teach your theory to your reader; you simply have to explain how you are applying it to your particular object. In an important sense, your theory sections constructs the objectivity of your object, it establishes a space of possible arrangements of things in the world, it “frames” your observations within this space. It is true that it identifies your “point of view” on your empirical material but it is no less true that this point of view is easily available to your readers. Their own training (in your discipline) allows them to easily step into it and see the world through your eyes. You share a perspective on the world and we call this perspective your “theory”.
The possible combinations of objects that a theory allows are, of course, infinite. But your theory section is a very finite piece of text, usually around five paragraphs long. So you have to select the subset of concepts from your shared theory that will most efficiently establish the perspective you need your reader to take. You must then present these concepts in a way that draws attention to the features of reality that your study has something to say about. If your study of Uber attempts to enter the Danish market construes it as “disruptive innovation” in the “sharing economy” you’ve already approached your topic in a particular way. You have constructed your object in a particular way, and one that now commits you to talk about, say, performance over time and redistribution markets. These are your theoretical commitments, we might say.
The implicit reader’s theory determines their expectations of your object. It is possible that, given what you say about the Copenhagen taxi industry (in you background section) the reader will expect Uber to have a hard time in Copenhagen. If your background includes the failure of Uber to operate in Copenhagen, the reader will expect your discover certain particular reasons for this, whether that be legal constraints that are lobbied for by an industry trying to avoid disruption, or market forces deriving from the trust that passengers have in the current system. (I don’t know anything about this in either theory or practice so I hope you’ll just let me throw these notions out there as formal examples.) Remember, however, that you are not just activating your reader’s expectations of the object. You are building a particular, formal expectation of you analysis. The theory tells us what your readers would have expected your analysis to show if they hadn’t read your introduction and therefore no better.
Putting it in terms more familiar to quantitative researchers, we can say that your theory, combined with your background, informs your null hypothesis or (for the Bayesians) your prior probability. It tells us what we would expect to find if the particular effect we are looking for isn’t playing a big role. Are we expecting the Copenhagen taxi market or the Danish policy environment to play the biggest role in determining the success of Uber there? What expectations does our theory of disruptive innovation lead us to have? This, at bottom, is what they theory section is supposed to make explicit.