Air is 70% nitrogen, 29% oxygen and 1% other gasses. It also includes impurities such as dirt, oil and water vapor.
Upon compression, water vapor will condense, mix with dirt and oil and can form an abrasive, often acidic, oily sludge.
Without air treatment, this murky mix will enter your compressed air system, corroding pipe work, damaging pneumatic tools & instrumentation and worst, compromise end product.
While air can be compressed into a smaller space, the amount of water vapor contained in the air remains the same raising the dew point. Dew point is the minimum temperature reached without condensation. It can be measured as atmospheric or at pressure