People need to travel. Whether it is to visit friends and relatives, enjoy a relaxing break away from home, or to do business in distant cities, air travel makes it possible. And transportation of goods by air brings to us the things that we need and want from around the globe. Demand for air transport will be driven by global economic growth and will, in turn, contribute to that growth. The need to expand the existing fleet to meet growing demand, plus the need to replace older airplanes with newer, better ones, creates a market for 27,210 new airplanes worth $2.6 trillion to be delivered over the next 20 years.
(Source: Boeing)

This stuff is really interesting: http://www.boeing.com/commercial/cmo/pdf/CMO_06.pdf