Racing exhaust systems?

OK, so i'm no expert when it comes to how the engine of a car works, but i was pondering a few things. When it comes to exhaust systems in racing engineered cars, if backpressure is such a huge factor when it comes to horses being generated, or lost for that reason, why do they have any exhaust system at all? Why bring a big pipe all the way to the back car to dispose of the wasted gasses if backpressure is such a factor? Theres gotta be something i'm missing about why you actually need an exhaust system on racecars, but if back pressure is an issue why not eliminate that by having no exhaust and just letting it go freely?

It's actually quite a bit more complicated than just backpressure. There is resonanace tuning, exhaust scavenging, etc... That and if you ran no exhuast the car would be so loud you'd start to lose your hearing pretty quickly. for a good race exhuast, that'll allow you to keep your hearing, i'd read this. http://www.se-r.net/car_info/engine_performance_tuning.html#Exhaust%20Systems
If the link doesn't send you there automatically, it's in the exhuast systems section. Hope this helps explain it better.