What does this do and is it necessary?

 

yes it VERY necessary.

Picture this, you have a turbo with exhausts gases going through it (spooling the turbo up) as the rpms/loads rise there is more exhuasts gases going through the turbo. This process could keep going infinitly and the turbo would "overspool" A wastegate helps control that enviroment. Once a certain PSI is acheved the wastegate opens and allows exhaust gases to bypass going through the turbo and just out the wastegate hole into either the enviroment or rerouted back into the exhaust (i.e. not letting the turbo overspool and become damaged).

 

Do you have any pics?

 

i guess to further clarify what was already stated...

http://www.streetracersonline....2.jpg

^ pic of an internal wastegate, which attaches directly to the turbo and releases through a hole right before your downpipe

http://us1.webpublications.com...i.jpg

^ pic of an external wastegate, which usually mounts on your manifold and releases either to the atmosphere or to a pipe which routes into your downpipe/exhaust.

basically when you've reached the desired amount of boost, the wastegate opens so that the exhaust doesnt spool the turbo to an undesired amount of boost. so it is very necessary!

hope that helps