Why did an HVAC installer add this PVC pipe?
That pipe looks like an air intake.

All "direct vent" style appliances (high efficiency on-demand water heater, gas fireplace, etc.) have an isolated air intake and it is common to have it suck in air from outside the building so as not to force air infiltration through doors, windows, etc.

Indeed, that is ugly.  Our fireplace and tankless water heater have something far more elegant:


The stack on the left is a natural gas tankless water heater.  The right is a direct vent natural gas fireplace insert.