True, diesels don't have throttles and sometimes (like my Powerstroke) don't have intakes. They just have a distributing plenum directly into the head ports.
The short answer is that intakes and exhaust are tuned to make proper use of the velocity carried by the mass of the airflow. Adding an exhaust can dramatically increase exhaust flow. I know it sounds backwards, but its not. The tuned exhaust diameter allows the exhaust air to speed up. If you just let the exhaust hit the surrounding air directly out of the ports, it stalls instantly. Putting a tube lets the exhaust achieve speed, which gives it inertia, which carries more exhaust.
Same for intakes. choosing intake runner length and diameter allows the incoming air to get up some steam, so that even after the piston has bottomed out on the intake stroke, the inertia of the incoming air packs in just a little more before the intake valve closes.
In the case of my diesel and other turbocharged engines, it doesn't need the intake's tuning to make proper power because its getting 27 psi from the turbo under almost all conditions
