This is a great question. I would say that reading of various theories helps in the process of reading and rereading. In other words, theories give a new perspective and with each new perspective, you are able to learn new things. Let me give you a few examples.
If you read the theories of Michel Foucault or Pierre Bourdieu, you will be able to look at power structures and see how power works through the literature. In other words, you gain insights on how to read texts from a new perspective.
If you read the theories of Mary Douglas, then you will gain a new grid to examine texts. Therefore, you will ask different questions, but this time based on cultural anthropology. If you read her works on symbols, then you may look at how symbols work in a culture.
If you read the works of Peter Berger, then you would look at a text from a sociological point of view.
In light of these example, you can see that the reading of theory gives the reader the ability to look at the same text with a new set of eyes.