because society demands it
no longer can someone choose to opt-out of society by going to live in the mountains, now you're forced to live in a communal environment whether you want to or not, until you're sufficiently rich enough ot leave the community. even then, the government will come by ever year to collect "rent" (tax) so that you cna live. that is why you need money. because other people say you do, and if you don't, you'll wind up in jail or dead.
It's a standard currency used in exchange for things everyone needs. You can acquire what you want or need and there is a set value for the money that allows everyone to trade equally.