What are kidney scans used for?

Multiple reasons: Kidney scans are used to diagnose kidney diseases such as kidney stones, obstructed kidneys, kidney cancer, and overall kidney function.

Many things: You didn't mention whether it is a ct scan, MRI scan, nuclear medicine scan or ultrasound. While the specific scans show slightly different things there is much overlap. In general they study kidney contour, size, function, obstruction masses and position.