it's complicated.
think of a witness as something like a city councilman, a computer tech, a lawyer, an economists, and other things...all rolled into one.
.
you have 30 votes for witnesses...with which you pick the BEST people who you want to represent you.

Hard to do...since most of what they do is incomprehsible to non technical people.
SO...delegate your witness proxy vote to someone who DOES know that stuff.

I've delegated MY proxy to @dragosroua
go to his blog...read some of his posts..
they explain it better than I can.