Someone wants something done, and it has certain value to them. The person delivering it values their time, and they might want money regardless of their feelings about that work. Maybe the money is part of what makes them happy to do the work.

Still, there certainly is a lot of unpaid work happening - open source projects, indie games, personal learning projects...

Labor Theory of Value:

https://www.investopedia.com/terms/l/labor-theory-of-value.a...

The pay is part of the reason why work is so terrific.

Because life is good.