As we all know and have heard many times the engine is fundamentally nothing more than an air pump. At the exhaust, turbulence and backpressure can adversely affect performance, especially at the high end of the power band.
Most headers are designed to improve performance at the high end of the rpm band. For an exhaust gas pressure pulse to move, the leading edge must be at a higher pressure than the surrounding pressure. The main body of the pulse is essentially at the surrounding pressure while the trailing edge of the pulse is at a partial vacuum. As the exhaust gas flow and associated pressure pulse moves through an individual header tube, the pulse loses energy and gas flow slows down. Once the pressure pulse reaches the collector it no longer has the energy to cause significat backpressure at the cylinder head. Thus, the next pulse is allowed to enter the tube with little restriction.
A properly designed exhaust system will take advantage of the vacuum created by each pulse to help scavenge subsequent exhaust gasses from the cylinders. This is one of the primary basics in exhaust header tuning, i.e. varying the length of the primary header pipe in order to time the vacuum generated by a previous pulse, in relation to cam overlap, to help to pull in a fresh intake charge while completely removing (at least in theory) the residual exhaust gases.
One of the misconceptions that many have with regard to exhaust systems is “that a larger diameter is always better”. Primary tube as well as exhaust pipe diameters are optimized (not necessarily made larger) depending on what part of the power band you are trying to optimize. Larger diameters allow the gases to expand, resulting in gas cooling and a slower gas flow. In conjunction with a cam with a lot of overlap, this may be what you want for a racing application where the engine is always operating in the upper rpm band. In general, this would not be the way to optimize low end torque. If low end torque and “cruising streetability” is what you are after, stock exhaust manifolds are not too bad a choice.