Why shares outstanding is useful
A company's number of shares outstanding is used to calculate many widely used financial metrics. Market capitalization -- share price times number of shares outstanding -- and EPS are both computed using a company's number of outstanding shares.
Recognizing that a company's number of shares outstanding can change is also useful. For example, the difference between the number of shares currently outstanding and the number of shares fully diluted is comparatively likely to be significant for fast-growing technology companies. These companies aggressively fund their growth by using convertible debt and paying employees with stock incentives. By contrast, many older stalwart companies are likely to have a number of shares outstanding that matches its number of shares fully diluted.