• Allow people geographically separated to exchange goods and services.
• Provide an incentive for other people to do work that you want done, especially work that is unpleasant or work you can’t do for yourself.
• Permit you to do the work that you enjoy and allow others to do the work for you that they enjoy doing.
• Ensure scarce resources are reserved for high priority uses.
• Encourage a large market for the resources that are easy for you to reproduce.
• Easily keep track of who owes you even very small favours, where favours are transferable and accumulative to facilitate trade.
• Track negative favours, or torts, so that there is a way for other people to apologise to you for the harm they do you and to discourage them from repeating the offense.
• Encourage the invention of new ways to produce scarce resources and to find alternatives so that you always have a sufficiency.
• Provide for orderly, fair competition and natural selection between various genetic strains of the human species.
• Help enforce social conventions involving respect and status so that others treat you with the respect you deserve even when they don’t know you personally.
• Prevent cheating on the agreed upon rules.

One way to stimulate your imagination to invent new forms of digital cash is to imagine an alien watching a documentary about human activities. To help get you into the alien frame of mind, imagine that the images have been electronically retouched to erase all the price tags, money, credit cards and replace them with blank white objects or perfectly transparent objects. All the aliens see are the physical actions. They don’t see or understand money. The aliens speculate on motivations for the various human activities. If nothing else, you may get a chuckle. I wrote a little scenario on bix.com in technology/social;5074 about how aliens might look at the social rituals involved in buying fruits and vegetables.