Why we use PNP power transistor in Isolation circuit? ?

The signal from optocoupler is first given to PNP power transistor


before giving it to Relay. Why is it so?Why we use PNP power transistor in Isolation circuit? ?
The difference is in the current capacity. The relay draws too much current for the opto device to drive directly, so the power transistor is used to amplify the current handling capability.Think of the power transistor as an electronic lever with a small signal driving a much larger signal which drives the even bigger relay which can handle even larger current and voltage. The relay contacts are isolated from the transistor circuits and the power transistor isolates the relay coil from the optoisolator output circuit, and finally, the optoisolator uses a light beam to signal whether an object is in a certain location or not without a physical contact to ';feel'; the presence.