And this is better than I could explain;
You will definitely feel this one. The stock plenum slopes down towards the front of the engine. Nissan designed this in order to clear the stock strut bar and hood on the 350Z (function after form). This sloping design restricts the airflow to the front cylinders, but an aftermarket plenum can help the front cylinders breathe. Aftermarket plenum spacers either increase the overall height or remove the downward slope in the design, allowing more volume and greater airflow to all cylinders. Higher airflow is needed to achieve more power.
Note: Some plenum spacers will cause you to loose the stock strut bar.