What does the diode / zener diode protection circuit actually do? What happens if I lose it?

Cheers!

Zeners are usually used for overvoltage protection (connected in reverse in parallel with the load). The zeners are selected to match the load and conduct when the desired load voltage is exceeded. I don't know the exact circuit your talking about but I would definitely keep it or things could start going bang.

[Edited on 2/12/12 by wylliezx9r]