The purpose of money should be to encourage the exchange of labor,
Exchange of labor is one purpose.
More generally that purpose is for any exchange in general, thus avoiding problems caused by missing coincidence of wants.
Another purpose is to store value, such as from my labor now to protect myself against a future time when I am unable to labor.
Another purpose is as an aid in accounting, such as in determining the relative value of various labors I might perform, directly useful in determining if I would rather have someone to do a job for me or do it myself. Or in simple bookkeeping, money being one of those revolutionary ideas along with double-entry accounting which revolutionized business record keeping. Both of those are key factors enabling specialization, a certain amount of which is highly beneficial for all of society as well as individuals.
Those three purposes are essentially all concerned with the 'now' uses of money. The fourth is as a standard of value over time, to enable comparison and choice not only between items now but between now and the past or the future.
A little ditty that used to be taught to help children remember the purpose of money: "Money is a matter of functions four: a medium, a measure, a standard, a store." Meaning a medium for the exchange of value, a measure of value, a standard of value, and a store of value.