The purpose of a turn-and-slip indicator is to:

The purpose of a turn-and-slip indicator is primarily to determine the quality of the turn. This instrument provides pilots with information about the coordination of the aircraft's turning maneuver. Specifically, it indicates whether the aircraft is in a coordinated turn, which means that the forces acting on it are balanced.

When an aircraft makes a turn, it is important for the pilot to maintain a proper bank angle and ensure that the turn is neither too steep nor too shallow. The turn-and-slip indicator shows whether the aircraft is rolling out correctly into the turn—if the aircraft is skidding or rolling. A properly coordinated turn should keep the ball centered in the indicator. If the ball is displaced to one side, it alerts the pilot that the turn is uncoordinated, which can lead to risks like rolling out of the turn too steeply or experiencing increased yaw which may affect handling and performance.

Other options like indicating the rate of ascent, showing the aircraft's heading, or measuring the attitude do not fall under the function of a turn-and-slip indicator. Each of these parameters is measured by different instruments, such as the altimeter for ascent rates, the heading indicator or magnetic compass for heading, and the attitude indicator for the aircraft's orientation relative to the horizon