Several studies have shown that "learner-generated external visual representations" (LGVRs) play a vital role for learning scientific concepts and processes. However, we know little about teachers' reasoning as to why and how to implement LGVR activities in their teaching. Thus, the purpose of our study is to determine why teachers let their students construct visual representations in biology, what benefits they see in these activities, and how they encounter difficulties that show up while students draw. We adopted the theoretical framework of the theory of planned behavior and the three-component model of attitudes to gain insights into teachers' reasoning when it comes to LGVR activities. According to this framework, we focused our analysis on five constructs (the attitude towards the behavior with its affective, behavioral, and cognitive component, the perceived behavioral control, and the subjective norm) and conducted interviews with six pre-service teachers and seven in-service biology teachers. We found that the first four constructs of our theoretical framework, but not the subjective norm, have a substantial influence on the teachers' intention to utilize LGVR activities in class. It became clear that teachers mainly focus on the construction of realistic pictures, whereas learner-generated abstract pictures play a minor role. Besides this, we achieved an informative insight into teachers' reasoning about the use or non-use of drawings in specific classroom situations (e.g., diagnosis, assessment).