The 2001-2002 3.5 V6 models produce 240 hp (180 kW) and 245 lb·ft of torque from 3000 to 5000 rpm. 2003-2006 model engine produces 265 hp (198 kW) and 253 lb·ft, this was done largely via modifications to the exhaust system to improve airflow. KnN filters are designed to increase such airflow and give more to the engine when needed. The end result is more horse power, better response and better mpg.
K and N designs their air filters to provide minimum restriction allowing high airflow into an engine; increased airflow will increase engine performance measured by horsepower and throttle response (torque).
K and N filters are also reusable so you never have to replace your air filter again; just clean it.