I'm not an engineer, but for what it is worth. The amount of backpressure affects the performance of the engine, both overall horseposer and torque, and the amount of each at a given RPM. A term you may hear is a "tuned exhaust." It is done in lots of ways: pipe diameter, pipe length, mufflers, etc. On race cars (e.g., NASCAR) there are often multiple pieces of exhaust pipe of different lengths and diameters to get the desired torque and horsepower "curves." An important piece is having the same effective pipe length for each cylinder, hence the funny contorted bends you often see in aftermarket headers. I've seen a couple of books that explain all of the math needed to do the job right. Made my head hurt.
Kevin