My understanding - which will probably be shot down in flames, but at least i'll be put right by the amount of knowledge on here - is dump valves were originally designed for very high performance engines running huge turbos with big boost figures.

Dump valves - atmospheric as opposed to recirculating were designed to alleviate the pressure on the system as high boost was unsuitable when a gear change happens. I've heard rumour of it stopping the fins on a turbo from shattering under high pressures but i honestly thing that's twaddle - either that or a very early problem within turbo development.

As for the sound. you can get valves that recirculate and give an atmospheric dump but i'm sure die-hard performance enthusiasts will lean towards completely recirc valves for obvious reasons.

I didn't realise there was so much of an anti-atmospheric feeling on here.