Theories have several basic functions; they are used to describe, to explain, and to predict.
The role of theory in your dissertation is to provide an organizing framework through which you conduct your study.
In addition, your dissertation research can make a contribution to the confidence we have in a particular theory or to our understanding of the conditions under which the theory coheres.
The basic purpose of scientific research is theory.
A research question is of little consequence without a theoretical framework to help conceptualize why this should be so.
One of the contexts that your review of the literature provides for your research is the theoretical connection between variables of interest. This connection allows you to make tentative predictions and test how robust the theory may be.
The Dissertation: From Beginning to End
Peter Lyons & Howard J. Douek