What is the purpose of a filter-drier in a refrigeration system?

The concept behind this question

The refrigeration cycle moves heat rather than creating cold, and every question about pressures and temperatures comes back to it. The compressor raises the refrigerant's pressure and temperature; the condenser rejects heat and the refrigerant becomes liquid; the metering device drops the pressure; the evaporator absorbs heat from the space and the refrigerant boils back to vapour. Because pressure and temperature are related for a saturated refrigerant, a pressure reading plus a P/T chart tells you the saturation temperature — which is the basis for measuring superheat and subcooling.