Well, besides everyone being lazy without it, it solves problems. See, if people want something, they can get it from other people, but said people usualy want something in return, and sometimes the first person does have that something and...
Well, this is getting confusing. Let's say there's two people. Person A has a hat, and Person B wants the hat, but they only have a Pogo Stick. Now, Person A doesn't want a pogo stick, they want their windows cleaned. Without money, what can you do? Trade the pogo stick for services? What if the window cleaner doesn't want a pogo stick?
But incert money, and it all becomes a lot less luck based. It goes something like this:
1) Person B finds someone who wants a pogo stick, and has some money, and they trade.
2) Person B gives Person A the money in return for the hat.
3) Person A gives a window cleaner the money for services.
See? Much simpler.
Plus, Money is small. If we didn't have money, there's a chance you'd have to bring cows and goats with you into the supermarket to get your weekly shopping, and times that by the other 100 people who need shopping, and... Well.