If stock prices were to fall precipitously from here, many companies which have been previously overpriced, would become cheaper. As a result, investors would be able to scoop up great businesses at depressed prices. Those who are still in the accumulation stage and deploy their cash every month will be able to take advantage of the opportunity and buy quality shares at a discount.
More experienced investors might also decide to use margin and make investments, a few days or weeks prior to any planned cash deposits into their brokerage accounts. Investors who held some cash however, would likely be able to deploy a large portion of it in attractively valued stocks, provided that they do not freeze under fire, and provided that they do not wait for even lower prices that may or may not materialize.
These are the investors who should carefully consider however the opportunity cost of holding that cash for extended periods of time waiting for a market decline, versus deploying that cash immediately.
Full Disclosure: Long MCD, KO, PM, MO, KMI, YUM,