Dump valves were invented by Saab in the 70's as a Noise Surpressant device. The chatter was deemed to annoying for road applications, so they fitted a reciring d/v.

Do they do anything for the preformace, turbo life or reliability of your car?

Nope.

Do they sound stupid?

Yup.

Dump valves are also really bad for the MFi Escort Turbo. You'll notice the car runs alot better without it.