The replication crisis-a failure to replicate foundational studies-has sparked a conversation in psychology, HCI, and beyond about scientific reliability. To address the crisis, researchers increasingly adopt preregistration: the practice of documenting research plans before conducting a study. Done properly, preregistration should reduce bias from taking exploratory findings as confirmatory. It is crucial to treat preregistration, often an online form/template, as a user-centered design problem to ensure preregistration achieves its intended goal. To understand preregistration in practice, we conducted 14 semi-structured interviews with preregistration users (researchers) who ranged in seniority and experience. We identified two main purposes researchers have for using preregistration in addition to different user roles and adoption barriers. With the ultimate goal of improving the reliability of scientific findings, we suggest opportunities to explicitly support the different aspects of preregistration use based on our findings.