Isaac has the basic idea. The purpose is to make your Intake
ports, whether it be a 4 cyl., 6cyl., or 8 cyl., flow more effectively. If the air ports through with ease, not having to turn corners or travel along rough edges, you'll get more air in the cylinder which means more HP and Tq.
Port and Polish is like opening your nostrils wider and bigger to breather better. You can't breath well when you have a stuffy nose and nor can an engine.
Port and Polishing might alter the Air/Fuel ratios but that can be corrected by an Air/Fuel Controller. Apex-i and I believe Blitz makes these controllers. These air/fuel controllers need to be dyno tuned however.
If you do decide to get your Intake
manifold's ports polished make sure you do it at a reputable shop. They would have to perform a flow test to see if each cylinder is receiving the same amount of air or else it will cause you to run rich or lean in various cylinders.
Also, get your hands on a separate head so that the shop can practice on it and recreate it on your cars actual heads.
Hope that clears things up