If theory doesn't help you understand yourself and the world around you, it's not worth doing. People come up with these ideas because they help them solve problems, and we should use them to do the same. We turn to theory to explore and better understand these questions. Interpretations of literature are never definitively right or wrong, merely plausible or implausible in a given context. Our task as theorists is to shape our ideas about literature in a way that other people who have thought about it find plausible and interesting.
Most critics use ideas from a variety of backgrounds to try to understand a text.
Some paradigms for thinking about literature are so pervasive that most influential critics publishing today take them as a point of departure. Deconstruction and poststructuralism fit into this category. Similarly, most critics would agree that issues like gender, first raised by feminist critics, or social class, first raised by Marxist critics, are important to understanding literature, even if there is spirited disagreement about how best to explore the issues.