Money is a claim on the future efforts of the people on this planet. If you have $500 and the average wage is $25 per hour, you can enjoy the fruits of 20 average man hours worth of product. That might be 20 hours at the barber for example. Or 20 babysitter hours. Or if you buy 50 T shirts for $10 each, you are purchasing the thousands of slices of man minutes embedded in what it cost to make the shirt, ship it and put it in a store. If you have enough money, you can shift the occupations people choose. If you had $10 billion, lived in a small town and you decided you wanted to live a life of debauchery and spend all your money, you could shift the employment patterns of that small town considerably. Many will change their jobs to the liquor, escort, party planning, domestic servant industry. Or you could endow a charity devoted to studying cancer, and commit to spend your $10B, thereby causing an uptick in people entering the cancer research world. Like Warren Buffett liked to say, if he wanted he could have a army of painters paint his portrait every day for decades. So money is a claim on society's efforts, and if you have enough of it you can influence a part of society what kind of efforts they should direct themselves towards. Note that you don't have to be rich to achieve this - the Pharoah of Egypt probably didn't have a paycheck, but he could direct thousands of man hours to build pyramids.
On a personal level its labor. If labor was free everything would be free and there would be no need for money.
The function of money is so that trade can be made easier. If I wanted some chicken eggs and the guy selling the eggs only accepted apples for the eggs and I only had oranges, I would have to spend my time finding someone with apples to give my oranges just so I could give those apples to the guy with eggs. A totally inefficient system. Money makes our life so much easier.
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