What is the Function of Pressurization Light on Exit Doors for Commercial Airliners?
It is there to prevent "violent door openings":



(figure 1b source)


  In some abnormal cases, outflow valves can remain closed, when the
  aircraft is on the ground, causing the air pressure in the cabin to be
  higher than the ambient air pressure outside the aircraft (fig.1b). In
  this case, there is a risk that an aircraft door could violently open
  and injure the operator or damage the aircraft.



(fig 2 source)


  The Residual Pressure Warning Light (fig.2), part of the residual pressure
  warning function, warns anyone who wants to open a door if the
  aircraft is pressurized. This device is installed on all Airbus
  aircraft except for some A300, A310 and A300-600 where it was offered
  as an option. On these aircraft not fitted with the warning light, a
  caution placard located on the door reminds the operator of the risk
  that residual pressure may cause violent door opening.


This brief from Airbus has a lot of good info on residual cabin pressure as well. 



Not to be confused with the "Slide Armed" light.