Social studies of knowledge have contributed many insights into the making and the character of scientific and technological knowledge. However, studies of knowledge use are scarce. This article engages with under-examined topics concerning epistemic utility. I posit and demonstrate that scientific and technological knowledge claims are functional. I argue that knowledge and its functions are mutually-enabling and mutually-sustaining constructs. To substantiate my claims, I present useful conceptualisations of ‘function’ and ‘functionality’ and employ them in an empirical case study. I examine (...) knowledge use in synthetic biology, a young form of biological engineering. I demonstrate that knowledge in the field is brought into existence with function in mind, kept in existence through functional use, qualified and situated by its functionality and evaluated by its functional operation. The case study reveals the effectiveness of my conceptualisations and offers lessons about knowledge as both an end-result of epistemic work and a mechanism for practice. My theoretical and empirical contributions expand understanding of scientific and technological knowledge and shed light on an area of study awaiting investigation. I conclude with open-ended reflections on work yet to be done. (shrink)