Previous work has shown that many physics students will draw unprompted diagrams while solving physics problems. These diagrams are distinct from prompted student-generated diagrams in that they tend to be smaller and contain fewer details, an unsurprising finding given that these two types of diagrams are generated for different reasons. In this study, we expand and elaborate on reasons physics problem-solvers generate diagrams and the differences between unprompted and prompted diagrams by analyzing a set of 10 interviews with undergraduates, graduates, postdocs, and faculty in physics. Interviewees were given several physics problems and diagramming tasks to anchor a subsequent conversation about diagramming while problem solving. In these conversations, participants reported that their unprompted diagrams help them: understand the situation and question; store, organize, and prioritize information; support solution strategies; and communicate information. Participants also shared their beliefs about the limitations of diagrams, including when they are not useful or stop being useful, as well as difficulties with generating diagrams.