Business Owners play a critical role in large-scale data programmes, serving as key decision-makers who ensure strategic alignment, prioritize value delivery, and maintain business accountability across Agile Release Trains. This paper explores the significance of Business Ownership in complex data programmes, where cross-functional collaboration, regulatory constraints, and interdependent data products present unique delivery challenges. It examines how Business Owners influence incremental and iterative development, from shaping PI objectives and prioritizing features to resolving blockers, coordinating dependencies, and validating value through system demos and Inspect & Adapt events. Drawing on a real-world case study within a leading fund and corporate services financial institution, this paper presents a practical implementation of Business Ownership in a large-scale data programme. It highlights how Business Owners were structured across business areas, integrated into delivery workflows, and empowered to drive scope, release planning, and risk-based decision-making. The findings underscore the importance of embedding Business Owners not only at the strategic layer but also in day-to-day agile execution to support adaptive planning and ensure business value realization. This study contributes to the growing body of knowledge on Lean-Agile governance in enterprise data initiatives, emphasizing Business Ownership as a cornerstone of sustainable and value-driven agility at scale.