Power Query is a critical tool for Power BI developers as it allows them to clean, transform, and load data from various sources into Power BI. Power BI developers use Power Query to filter out unnecessary data, merge datasets, and shape data into the desired format before visualization. Mastering Power Query helps developers automate repetitive data preparation tasks, ensuring that reports remain updated with the latest data. By streamlining data processing, Power Query enhances the efficiency and accuracy of Power BI solutions.