In short, you need the economists' assumption that trade occurs at the equilibrium.
If you know the supply and the demand, and if you are willing to assume that trade occurs at the equilibrium (at least eventually, after buyers and sellers have adapted to changes in the market), then you have a sharp prediction: the price and quantity will be the equilibrium price and quantity.
We have seen in class that this does not always need to be true. We have talked about a number of examples where:
- trade does not happen at the equilibrium, and
- trade that starts outside of the equilibrium does not converge to the equilibrium.
Yet, we have also discussed why the equilibrium remains economists' best guess given a certain supply and a certain demand.
If the equilibrium assumption is deemed unsatisfactory, then we have to replace it by something else. Many economists have tried, and keep trying. But finding an alternative hypothesis that does better than the equilibrium hypothesis turns out to be a difficult task (some economists even think that the equilibrium hypothesis is the best hypothesis there is, and that it is pointless to try to improve upon it).