In pull-based development model, code contributions are submitted as pull requests (PRs) to receive a review by other developers before the PRs are merged into the code base. The effectiveness of this development model is dependent on a proper understanding of the submitted PRs by developers reviewing them. Although practitioner guidelines and community standards emphasize best practices for writing PR descriptions, the actual impact of the content of PR descriptions on the code review process has not been empirically evaluated. This paper presents a mixed-methods empirical investigation into the value of PR descriptions. We performed a grey literature review of guidelines for writing effective PR descriptions and derived a taxonomy of eight PR description elements-purpose of the PR, explanation of code changes, type of feedback requested, etc. Building on this taxonomy, we analyzed 80K PRs from 156 GitHub projects and employed regression modeling to examine how the presence of each element in PR descriptions relates to code review outcomes such as PR merge decision. Likewise, we sought to understand how developers perceive the importance of these elements and conducted an online survey with 64 developers to assess how practitioners value different PR description elements. Our results show that developers broadly perceive PR descriptions as important and descriptive elements—those explaining the purpose, code changes, and testing—are regarded as important in most reviews, supporting understanding and traceability. However, regression modeling reveals that elements focused on interaction with the reviewers-such as specifying the type of feedback requested-are related to a higher likelihood of a PR being merged and promote higher review engagement, despite being perceived as important in fewer reviews by developers. To understand the context in which descriptions gain importance, we find that PR descriptions are more common in mature projects and complex code changes, suggesting that description writing is an adaptive, context-sensitive behavior. Overall, our findings highlight that PR descriptions function not only as explanatory artifacts but also as coordination devices that shape reviewer interaction, underscoring the need for contribution guidelines and review tools that explicitly encourage both descriptive clarity and interaction-oriented communication in PR descriptions.