Although lesson study has been shown to improve the teaching and learning of mathematics, little research has focused on reasons for engaging in it from a teacher's perspective, particularly after sustained engagement with lesson study. This qualitative study reports on thirty-three teachers from the United States who were involved in twelve lesson study cycles during a three-year partnership aimed at improving algebraic instruction for students aged 8-14 years during a period of educational reform. It used grounded theory techniques to analyse interview data to determine the purposes of lesson study from teachers' perspectives. Results highlight six purposes for engaging in a mathematics lesson study: understand reform standards, focus on student thinking, develop pedagogy, support collaboration, develop curricular materials, and learn mathematics. Particularly noticeable was its use for focusing on student mathematical thinking and understanding of reform initiatives and requirements. Research implications suggest positioning lesson study as a useful, teacher-vetted tool for unpacking educational reforms.