The typical reason for replacing a contactor when the system is still running is the contacts are badly burned and pitted. They will start sticking. This means the compressor will not shut off when told to by the thermostat. Refrigerant in a liquid state does not compress. This can damage a compressor as it will try and compress liquid.
A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings. A contactor is controlled by a circuit which has a much lower power level than the switched circuit.
Should you need additional information on contactors, give us a call 239-596-5855.