Your question is really good! I agree with what akannan has posted and want to add a few thoughts of my own.
First, yes, I think that to interpret literature requires some sort of approach to literature beyond simply understanding the story, whether or not that is a formalized approach.
Second, for me, theory is valuable because it can encourage us to move past our initial ways of understanding a work. Rather that look for expressions of some universal human experience or some parallel between the author's life and the author's work, for example, we can value a work for how it presents class conflicts or structures the reader's experience or whatever.
In my view, most readers do not think much about how they arrive at the meaning of a literary work. For me, theory's real value lies in forcing us to think about how, where, and why we find meaning where we do.
I think that the term "only" naturally causes me to pose questions. I think that the interpretation of literature can happen on different levels. Part of what makes this fascinating is that the experience of interpreting literature might transpire on a personalized notion of experience that would make it one that could transcend literary theories. At the same time, there are so many literary theories that are present that they help to describe the experience of reading literature. They might not be the only ways to interpret it, but they might be tools developed from others' experience of it. This would mean that there can be expressions and experiences of literature that have not been articulated by literary theory, which could be more of a statement as to what has been experienced not as much as what should be experienced. Theory, in this light, is preceded by experience and reality, meaning that what could be experienced or understood has not been articulated by theory as of yet. In this light, literary theories are one way or set of ways to understand literature, but by no means the only way.