Window functions in SQL Server are a powerful tool for performing calculations over a set of rows related to a specific row. Unlike aggregate functions (like SUM, AVG), which collapse data into a single row per group, window functions keep the original rows intact while performing calculations over a defined window. This is particularly useful for tasks like calculating running totals, finding the rank of a value within a group, or partitioning data based on specific criteria.Imagine you have a sales table tracking daily sales figures. You might want to calculate the running total of sales for each day. Using a window function, you can achieve this without grouping the data by day. This allows you to see the cumulative sales for each day alongside the individual daily sales figures.Another common use case is ranking customers based on their total spending. A window function can assign a rank to each customer based on their spending, without requiring a separate ranking table or complex subqueries.Window functions are particularly useful when you need to perform calculations that involve multiple rows but don't want to lose the individual row data. They are a key part of analytical queries and provide a flexible way to analyze data.