Air flow restriction.
When the fins and air passages of an evaporator are clogged with dirt, air will have a difficult time passing through the evaporator. Since the principles of air conditioning require the heated air around the evaporator to lose heat, a vehicle with a clogged evaporator will have poor A/C performance.
Evaporator core maintenance.
The evaporator core should be removed from the case to properly clean it. Airborne debris, such as leaves, lint, hair, and other contaminants, enters the evaporator case and clings to the wet evaporator core.
The A/C drainage.
The drain is for the purpose of removing the condensation that forms as a result of humidity being removed from the intake air. It is nearly impossible to adequately clean an evaporator core without first removing it from the case.