This study investigates how in-service teachers perceive analyzing their own lower secondary students' written argumentation as useful. The participants were lower secondary school teachers enrolled in a university mathematics teaching program. The teachers planned and conducted a teaching lesson and wrote a report concerning the argumentation in their students' written work. The qualitative data consist of reports from in-service teachers and transcriptions of five follow-up interviews. Findings suggest that the teachers perceived the analysis useful for gaining insight into various parts of the students' prerequisites, establishing a basis for facilitating instruction, and acquiring knowledge related to different aspects of proof. The results imply that analyzing students' argumentation can be a meaningful and useful activity for in-service teachers to engage in.