The primary purpose of a purge valve is to release trapped air or gaseous nitrous from feed line(s). Helps to ensure consistent performances. And it looks cool, too.
i have one for sale , NOS company ,
Illuminated Dual LED Nitrous Purge Kit
http://www.holley.com/16037NOS.asp
brand new box packed 12k.
some vids , same kit is also installed in my car..
http://www.youtube.com/watch?v=554mjQQQEZQ
http://www.youtube.com/watch?v=ZfG6ZM3dlKM