Positive feedback has known benefits for improving task performance, but it is not clear why. On the one hand, positive feedback may direct attention to the task and one's motivations for having performed the task. On the other hand, positive feedback may direct attention to the task's value for achieving a future goal. This ambiguity presents a challenge for the design of automated feedback interventions. Specifically, it is unclear whether positive feedback will more effectively influence behavior when it praises the recipient for having performed an action, or when it highlights the action's value toward a goal. In the present study, we test these competing approaches in a large-scale field experiment (n = 1,766). Using a mobile app, we assigned college students to receive occasional notifications immediately upon submitting online assignments that either praised them for having submitted their coursework, or that highlighted the value of submitting coursework for academic success, or to a no-treatment control group. We find that only praise messages improved submission rates and course performance, suggesting that drawing attention to the feedback-eliciting task is necessary and sufficient for influencing behavior at scale.