我想問一問Earning Forecasts 有什麼好處和壞處呢?

Projection of Earnings or Earnings Per Share (EPS) frequently made by management and independent security analysts. Examples of forecast sources include (1) Lynch, Jones and Ryan's Institutional Brokers Estimate System (IBES), (2) Standard & Poor's The Earnings Forecaster, and (3) Zacks Investment Research's Icarus Service.
Sooner or later, a company stock price reflects its earning power -- or, at least, the market's perception of its earning power. So if you are a shareholder, most anything that an analyst -- or better yet, the company itself -- says about the firm's profit prospects should be of interest.
Pay particular attention when a company, usually through its chief financial officer, offers public "guidance" on the accuracy of the consensus earnings estimates. If a company officer says he or she is "comfortable" with analysts' forecasts, that's usually a pretty good sign that if earnings are going to vary from the consensus estimate, it will be on the upside. And such positive earnings surprises are very bullish for a stock.
Sometimes, however, a company officer will go public to warn that analysts' projections are overly optimistic. This is usually enough to send the firm's stock down, since the stock's price presumably reflects a multiple that investors are willing to pay for those future earnings. Lower expectations for earnings equals lower stock price.
Companies aren't always entirely forthcoming about such things, of course. Analysts -- and the market as a whole -- occasionally get blind-sided when a firm announces financial results far poorer than would justify the current price of its stock. In those cases, the stock doesn't just dip -- it plummets.