[sdiy] PSU question.

[Theo]

Hi List,

Rather often its omitted, but some pp say it should _really_ be there.
I am talking the reverse diode over the in- and out-puts of a 3 leg
stabilizer IC.
Actually I am not completely clear about what this reverse diode is supposed
to do.
The only way the cap on the input can discharge is though the 3 leg isn't
it?
So what is the use of this diode, paranoia or the basis of good PSU design?
Not trolling here, I am really this stupid.

Guidance appreciated,
Theo