The big reason why you use free cash flow to value a stock instead of earnings is that free cash flow is what companies use to do the things that add value to their stock.
Free cash flow is cash that can be used to make acquisitions, pay dividends, or buy back stock. It can also be used to pay down debt. Which is sometimes needed. But it’s not something that adds a lot of value to the stock.
So the things we want to focus on are acquisitions, dividends, and stock buybacks. Those 3 things often decide whether or not you make money in a stock.