what is the purpose of them and when do u put them on

If your talking about an air intake under the air box, there is debate if it is actually of any benefit, where you attempt to collected extra air from is a low pressure zone anyway. Better off finding a tickford CAI, larger mouth more air, goes straight in without cutting anything.

You may also want to be a little more descriptive in your explanation as there are also a few variations of this, a few being sequential intakes, an extra air feed to the air box and twin throttle bodies. tickford intakes flow plenty, you should not need anything larger than that unless you have some serious mods.

nah i mean twin throttle bodies

pretty much only needed when massive amounts of air are being pushed in, ie turbo applications. standard TB flows plenty, they are not common and a machine shop would need to fab one up for you, so not cheap either. the only ones that exist are all aftermarket jobs and there are not may around, mostly its for a wank factor on a NA motor.

i sold my custom made "flipped up side down box car log manifild with twin throttlebody's to "xcabbi".. not worth the trouble unless you have a ballistic 6 and are prepared to do alot of tuning..

ok cool just seen a pic of one and wondered what they are for thanks

They are for insane amounts of power and not much else. How insane may you ask? I'll let you know when its all built.