Air cargo is used by global importers and exporters when they need to get goods somewhere rapidly and reliably. While 90% of everything is shipped by ocean, air connects the world faster, cutting China-US freight shipping time from 20-30 days by ocean to only 3 days by air cargo.
International air freight and express freight shipments are not the same thing. Express freight is typically handled by one company (like DHL, UPS or FedEx) that handles the entire shipment lifecycle, with shipping from door to door in under five days. These shipments are usually smaller (less than one cubic meter and 200 kilograms) than air freight.