Your comments

No details added from originator in 3 months since explanation was requested by CDuv, and also no comment from the 3 who voted. Makes me think that this is not so important...

(I did not vote. Not a feature I have missed.)

For what it is worth, the only explanation I can see is that Alex requests an option to make ANY field mandatory (registration not possible unless the selected fields satisfy some condition).

I suspect that this can be done by way of rules.