Power Query (M Language) is essential for data transformation, cleansing, and preparation before visualization. It allows developers to merge, append, and shape data from multiple sources, ensuring a clean dataset for analysis. Power Query automates data extraction from APIs, SQL databases, Excel, SharePoint, and cloud storage while applying transformations such as column filtering, pivoting, unpivoting, and custom calculations. Proficiency in M scripting enhances automation and improves data refresh efficiency. Power BI developers skilled in Power Query can create repeatable ETL processes, reduce manual data handling, and ensure high-quality data for accurate Power BI reporting.