Your PCM has in it maps for controlling fuel and spark (I'm simplifying it myself)... When you install a new camshaft, your motor's fuel requirement will change, and it will change different amounts at different rpms, depending on the cam profile. You need to tell the computer when it needs to add more fuel, and how much more. All because with a new cam you'll be increasing the airflow of the motor, or in other words, making it a more efficient air pump. At the same time, timing can be modified on any car. On a normally aspirated motor, you're changing the timing curve to add or remove timing at certain points in the band, to increase power.