Ontologies have become popular in a wide range of domains in recent years. Although there is a large number of ontologies in existence, a review on ontology literature indicates a lack of clarity on the notion of ontology usefulness. To understand how ontologies are used and what evidence exists for their usefulness, we conducted a systematic literature review of research to understand current thought and practice. Our analysis shows that ontologies have been used in many fields, with the use categorized into two settings: use in computer systems and use to support human tasks. While there is strong evidence for the usefulness of ontologies in computer systems, a paucity of research exists to clarify the usefulness of ontologies as tools to support human tasks, especially in settings related to sharing domain understanding (an oft cited benefit of ontology use). In addition, our analysis of methodologies for ontology construction also indicates that most methodologies lack prescribed activities for evaluation of usefulness. In this paper, we explore ontology use and evidence of usefulness, and propose research opportunities related to exploring ontology usefulness with respect to supporting human tasks.