When you first start an account you likely won't have very much money in it. If you leave it as cash or even in a money market it will just sit there, no matter what the stock market does. You could buy a stock in that account but then 100% of your investment is dependent on what that one company does. What if you accidentally choose an Enron or a Lehman brothers and it goes bankrupt? One way to avoid that risk is buy LOTS of companies- the odds of more than one or two going