Karen Nichols and Marion Sealey invited me to write the foreword to their collection The Politics of Theory and the Practice of Critical Librarianship, and I said yes before I had thought through what a foreword is for. Thanks to Library Juice Press, the chapters are openly deposited, so the argument is available to anyone who wants to read it rather than only to those whose libraries can afford the volume.
The question in the title is the one librarians ask most often about theory, usually with an edge to it. Why read Butler or Weber when the reference desk is busy and the catalogue is broken? My answer in the foreword is that theory is not a decoration on top of practice. It is the thing that lets you see the practice you are already doing: why a subject heading feels obvious, why a budget line looks natural, why one kind of labour is visible in an annual report and another kind is not.
Theory does not tell you what to do on Monday morning. It tells you that Monday morning could have been arranged otherwise, and that someone arranged it. That is enough to start from.