Suction filter dryers are typically installed on air conditioning systems that have had a compressor failure and when observed normally indicate that a compressor was replaced on the system. When a compressor motor fails it can introduce solid particles and acidify the refrigerant. This contamination can cause failure of various components of the system, including the thermal expansion valve or the newly installed compressor and require consideration to ensure the future performance of the system. Often a filter dryer is installed to remove any debris from the refrigerant system, which may have entered the system as a result of a compressor failure, and remove any acidity in the refrigerant that could have occurred due to a motor winding failure. This filter dryer is installed on the refrigerant line returning to the compressor and remains in place for several days or weeks after replacement of a compressor. Service ports exist at the inlet and discharge of the filter dryer to allow a technician to take pressure readings across the filter. If the filter has become clogged it can be replaced or removed to prevent the filter from causing a restriction in mass flow of refrigerant that will adversely affect the performance of the air conditioning system. In cases where there is no pressure drop the suction filter dryer can be left in place, but must be monitored and maintained as it is prone to corrosion failure due to weather and sweating of the dryer with normal use of the system. Some technicians do not use suction filter dryers and elect to flush the system and use refrigerant additives to achieve the same results and you may not see a filter dryer on the suction line after compressor replacement. Working with my father in my youth I observed he always flushed the system and put in a filter dryer, returning to remove it a week later as he felt that was best practice.
If your inspection points out a suction filter dryer on an air conditioning system you should have the system serviced by an air conditioning technician who can determine if the dryer should be removed or if it can remain in place, which may take into considering the pressure drop across the dryer as well as the anticipated remaining life of the system. Suction filters are just one of many tools we use when evaluating an HVAC system.