Airspeed Calibration alternate static source, what is this for?
This table is included because of the fact that the Alternate Static Port on 172s is inside the cabin. Thus, changes in cabin pressure (even really small ones) can have an impact on the static pressure that the system is using and hence on the indicated airspeed. 

The things the table mentions like windows and heat vents all influence the pressure in the cabin, and hence the airspeed. In reality, if you have to use the alt static port you aren't going to be pulling out the POH to correct the speed as it is likely very similar.