Untreated compressed air always contains contaminants because of the nature of the gas and how it is produced. The need for air treatment basically results from 3 characteristics of compressed air.
Added by the compressor installation through oil lubricated compressors (oil), adsorption dryers and activated carbon filters (dirt), piping network and vessels (pipescale).
Trash in, trash out: oil vapors from car exhausts and industrial processes, atmospheric dirt and micro-organisms get sucked in by the compressor. As with water, their concentration – and thus importance – increases significantly after compression.