What is the purpose of a Zener diode added to the output of a SMPS?
Zener diodes are provided at the output of SMPS for a couple of reasons:

1) Protection against regulation malfunction- say your voltage divider that serves as a voltage feedback is not soldered correctly.  The output voltage (buck converter, say) can then rise all the way to the input voltage, which could destroy a number of components.  Typical control these days is constant on-time or peak current control and such the output current is limited.  Having the zener diode at the output will clamp the voltage to a reasonable level, the SMPM chip will limit the current.

2) To protect against unintentional back-feed that could destroy the SMPS regulator chip and propagate further.

3) The Zener voltage is higher because the knee characteristic starts just around 6.2 V and is not all that precise.  20% up will make sure the leakage current is minimal.

Source:  power electronics engineer at General Electric.