SQL is fundamental for Power BI developers as it allows for efficient querying and manipulation of data stored in relational databases. Proficiency in SQL enables developers to extract, join, and aggregate data, which can then be imported into Power BI for visualization and analysis. It’s especially useful when working with complex data sets that require advanced filtering or data preparation. Understanding SQL also helps in optimizing queries for better performance and in creating stored procedures or views that serve as data sources in Power BI. Mastery of SQL ensures seamless integration of data, contributing to the overall effectiveness of the dashboards.