I'm reading into Fama-French 3- and 5-factor models. I notice that they use the returns from market portfolios to "predict" stock excess returns. But obviously we cannot know ahead of time the returns from these market portfolios, and the returns from the market portfolios already incorporate the returns from individual stocks. By the time we know the returns for those market portfolios, we'll also know the returns for all the stocks. So what's good are these models? It seems to me that the proper use of them is to explain returns, rather than predict, which means one cannot use them to trade, no?