Let's first define what an ideal currency would be.  Currency serves two purposes; it allows me to express a preference for one good or service over another, and it allows me to express time preference (that is, when I acquire or consume a good or service.)

All currencies must satisfy at least one of these purposes, and an ideal currency must satisfy both.

The good and service preference is what allows you to, possessing a dozen eggs from a chicken, to obtain a gallon of gasoline without finding someone who has gasoline and wants eggs.  That is, it is the ability to use the currency as a fungible intermediary between two goods and services, one of which you possess and the other of which you desire.  Without this function in an economy you have only barter and poor specialization, with it you have excellent specialization and a much-more-diverse economic picture.