The MPSA13 is an NPN Darlington Transistor, which means that it has two NPN transistors in one package. As indicated in the pinouts figure, the first transistor's emitter is connected to the base of the second transistor, and both collector pins are clubbed together.
Because of this feature, the transistor can have a very high gain, such as the MPSA13, which has a gain of 5000. When building audio amplifier circuits, this high gain can be beneficial. The transistor is also utilized in sensors and display drivers because a very modest current into the transistor's base causes the transistor to conduct.