Everybody has a socially established number that is objectively associated with them - their net worth.

They can show other people how much they have, and everyone will agree as to what the number is.

People:

1. can do something which subtracts from this number and adds to another person’s number.

2. desire to accumulate higher numbers.

Money is therefore used for two things - to store it (i.e maintain the number) and trading (subtracting/adding to this number).

-- an excerpt from It's Not About The Technology, It's About The Money (2016), its 2-minute version can be found here: https://2minutebitcoin.org/blog/bitcoin-is-about-the-money-not-the-blockchain-technology