What is the function and purpose of an A/C coil ? Is it used to cool off air or for heat exchange ? I was told to replace my old A/C system I also have to upgrade my coil. Why

Cooling off air would be one way to describe it, but it actually absorbs the heat from in the home and moves it outside to the condenser via the lineset. The reason you are being told that the coil needs to be replaced to update the system, is refrigerents have changed from r22 to r410 and the best way to convert system is to replace every component of the ac system. We are in the next new era of residential ac. Your installer is not pulling your leg. He is giving you the best advice for the current situation. He most likely told you that you will also need a complete new lineset. This also is the best way to go. Hope this clears things up a bit. Please let us know how things turn out.