Been discussing this at work with lots of different responses ranging from, it does nothing, it makes a nice noise, it stops lag and stops the turbo from stalling.

So, my question is, what is the benifit and why are they not fitted as standard?

 

Nothing.
Doesn't stop lag.

 

personally i think it slows the car dwn slightly when its dumping well atleast my baileys feels that way it does sound good though

 

with a vent to air type a Wosh noise, aside from that not alot.

 

They are fitted as standard, but OEM dump valves are re-circulating ones, rather than vent to atmosphere (pssshhh) ones