Air is "free" but the production of compressed air is not. It requires expensive electrical energy. The production of 1 m3 of compressed air costs on average 1.5 to 3 cents (assuming electricity price: 0.10 €/kWh).
Up to 30% of this expensively produced compressed air is lost due to leaks – always when the compressor is running!
Even leaks with a diameter of just 3 mm will cost around 2,800 € per year (8,000 operating hours, 6 bar, 0.10 €/kWh).
The energy-intensive generation of compressed air adversely affects the CO2 balance, even if the generated compressed air escapes unused.