Attending to students' motivation to adopt computation as a physics practice is essential in effectively integrating computation into physics education. Within the Communities of Practice (COP) framework, addressing this motivational need involves inducting students into the community's sense of joint enterprise, which includes physicists' motivating reasons for adopting computational practices. We used the COP framework to interview students and instructors in the semester after they completed a set of computationally integrated upper division physics courses. This timing allowed us to assess students' perceptions of computation after they were no longer required to engage in computation in their coursework. This article discusses the reasons for using computation in physics identified by these students and instructors. We find that the students saw computation as a normative physics practice, and that they identified reasons for using computation that are consistent with those held by their instructors and the broader physics community. However, we also observe differences in the ways that these instructors and students articulated these reasons: The instructors drew on their research experience with computation, while the students drew on experience from coursework; each student tended to focus their overall discussion on a smaller subset of reasons than the instructors did; and students tended to discuss reasons in isolation, while instructors tended to interweave multiple reasons. We interpret these differences based on the students' positions and trajectories within the physics community.