BB I beleve the spinner to provide a stabilizing effect on the air before it enters the inlets and thus allows a greater velocity.

A pressure cowl noted by Les relys on a greater sized outlet than the inlet (usually to the rear and bottom but not always) to cause an increase in the velocity of air moving through the system.

Here is an extreme example (not my airplane). Note the tiny inlet but roughly twice the outlets one on each side to the rear of the cylinder bonnet.

On A sport airplane such as a stick of some sort or one of my multi engine bashs where the engine are completely exposed all of that is moot and I love the simple spinner nuts.

John