If the powersupply voltage get somewhat higher than the zener voltage (around 0.7v more) it'll turn off the PNP transistor and shutdown the output.
This is usefull if your powersupply have an unknow output voltage or have a chance to get crazy and destroy your circuit, so you can use something like that to protect a microcontroller for example.
BUT, you'll have voltage drop across the transistors, so think about it if it's worth it...
