Dump valve on a diesel?

[306pc]

Hi.
I've recently bought 2 incomplete fmic set-ups and now have enough pipe to finish mine. Both sets came with dump valves. What physically will they do for a diesel set up?
(Waiting for "will make you sound like a chav" reply. Please don't bother. That's the only bit I know, although I think I would enjoy the irony.)
But for both to have them I'm thinking there must be a benefit?

Thanks.